Mount Pleasant Caldera
The Mount Pleasant Caldera is a north-south trending elliptical
feature with minimum dimensions of 13 by 34 km as outlined by regional
gravity and magnetic
maps. The northern half is concealed by overlying Middle
Mississippian and Pennsylvanian strata. The caldera is bounded to the
east and west by polydeformed Ordovician to Silurian turbiditic
metasedimentary rocks of the Digdeguash and Flume Ridge formations. Late
Silurian to Devonian granitic rocks of the Saint George Batholith form
part of the boundary along the southern margin of the caldera. Rocks
within the caldera comprise part of the Upper Devonian Piskahegan Group
that is divisible into exocaldera,
This virtual field trip of the Mount Pleasant Caldera contains:
- Maps showing the surface distribution of the Mount Pleasant Caldera,
and its constituent groups, formations, and intrusive rocks;
- A stratigraphic column of the rock units;
- Digital photos of the rock units from significant exposures;
- Links to the New Brunswick Stratigraphic Lexicon;
- A listing of Fieldtrip Stops that pertain to corresponding rock
- Information about resources in the area.
All graphic- and photo-images may be downloaded and used freely.
Questions regarding this virtual field trip or the Mount Pleasant Caldera
should be directed to
Regional Geology Map
Simplified geology map of the Mount Pleasant Caldera, southwestern New
Brunswick. Exocaldera, intracaldera and late-caldera fill sequences are
recognized; older and younger rocks are respectively coloured pale blue and
grey. Click the blue Group and Formation names in the map
legend for detailed descriptions. Numbered blue dots are
links to Field Trip Stop descriptions.
Representative stratigraphic columns of the Mount Pleasant Caldera.
Columns 7 to 9 are representative of the exocaldera sequence. Columns 2 to 6
are representative of the intracaldera sequence. Columns 1, 4, 5 & 7 are
representative of the late caldera-fill sequence. Click the blue
in the legend to view photographs of these units.
The Exocaldera Sequence, in ascending stratigraphic order, consists of
the Hoyt Station Basalt, Rothea Formation, South Oromocto Andesite,
Carrow Formation and Bailey Rock Rhyolite. The first and last units have
the least areal extent.
The Hoyt Station Basalt comprises at least two flow units. Minor
pebble- to cobble-conglomerate and lithic lapilli-tuff is associated
with this basalt.
The Rothea Formation includes a lower member consisting of unwelded,
but compacted, pumiceous lapilli-tuff and crystal tuff units, a middle
member which grades from nearly aphyric tuff at the base to crystal tuff
at the top (pyroxene(?) pseudomorphs typify this unit) and an upper
member consisting of a lower fine-grained redbed unit and an upper
lithic tuff unit.
The South Oromocto Andesite is composed of at least three flow units
with the basal flow being the most areally extensive and the only one
exhibiting porphyritic textures. Calcite veins and hematite bands near
the top reflect degassing of the flow interior.
The Carrow Formation is predominantly a fining-upward redbed unit
that grades from pebble- to cobble-conglomerate at the base to mudstone
with intercalated calcrete at the top. Toward the southwest, the
conglomerate contains abundant clasts of the Seelys (intracaldera
sequence) and Rothea formations, but to the northeast, metasedimentary
clasts predominate. In the lower part of this formation, an unwelded,
but highly compacted, pumiceous lapilli-tuff contains abundant pumice
fragments. Locally, a basalt and basalt-clast mudflow occurs near the
top of the formation. A spore locality from the upper part of the Carrow
Formation has yielded a precise Late Famennian age (McGregor and
The Bailey Rock Rhyolite is a porphyritic lava and, like other lavas,
is characterized by an absence of angular crystal fragments and pumice
pseudomorphs. In places this rhyolite is intrusive into older rock
units. It is unique because it crosses the boundary between the
Exocaldera and Intracaldera sequences. A saprolite separates the Bailey
Rock Rhyolite from the overlying Late Caldera-Fill Sequence.
The Intracaldera Sequence comprises, in ascending order, the Scoullar
Mountain Formation, Little Mount Pleasant Formation, Seelys Formation
and McDougall Brook Granite. In addition, there are felsic dykes and one
mafic dyke that intrude the Scoullar Mountain and Little Mount Pleasant
The Scoullar Mountain Formation is characterized by sedimentary
breccia and interbedded andesitic lavas; felsic pyroclastic rocks are
voluminous in places and one sandstone-conglomerate unit is present. The
sedimentary breccia is dominated by pebble- to boulder-size angular
metasedimentary clasts, and a few undeformed crystal tuff clasts that
contain about 1% altered biotite. A pumiceous lapilli-tuff near the
apparent top has about 1% amphibole and accessory apatite.
The Little Mount Pleasant Formation is composed of crystal tuff and
flow-banded rhyolite. The crystal tuff is characterized by unflattened
to weakly flattened, microcrystalline, recrystallized pumice fragments
and chloritized amphibole with associated apatite. Phenocrysts inside
these recrystallized pumice fragments are an order of magnitude larger
than those outside, indicating that significant mechanical breakage of
phenocrysts occurred during eruption.
The Seelys Formation, consists of lithic tuffs and pumice-bearing
lithic lapilli-tuffs; banded, pumiceous, crystal tuff; and densely
welded crystal tuff. The basal part contains clasts of Scoullar Mountain
andesite and Little Mount Pleasant Formation. Quartz and feldspar
phenocrysts increase in size and abundance from base to top in the upper
part of the sequence. Platy biotite is virtually absent but metamict
zircon is a common accessory in all units.
The McDougall Brook Granite consists mostly of porphyritic
monzogranite, a border phase feldspar (± quartz) porphyry, and minor
equigranular to subporphyritic, fine-grained quartz monzonite. The
groundmass grain-size of the porphyry, the size and abundance of
feldspar phenocrysts increase inward, away from the contact with country
rocks. Chloritized amphibole with associated apatite is the main
ferromagnesian mineral phase in all three units. Parts of the feldspar
porphyry are hydrothermally altered, and a small hydrothermal breccia or
diatreme cuts the microgranite.
The relative stratigraphic position of units in the Exocaldera and
Intracaldera sequences is based on the following observations:
- The upper part of the Rothea Formation consistently contains about
1% platy biotite pseudomorphs. The only intracaldera rocks with this
much biotite are volcanic clasts within sedimentary breccia of, and a
tuff unit near the apparent base of, the Scoullar Mountain Formation.
- Andesitic rocks occur only in two units: the South Oromocto Andesite
of the Exocaldera Sequence and the Scoullar Mountain Formation of the
- The Carrow Formation contains clasts from the Seelys Formation.
- The Bailey Rock Rhyolite, which occurs in both sequences, intrudes
and/or overlies the Carrow Formation but is intruded by, or grades into,
the McDougall Brook Granite.
Late Caldera-Fill Sequence
The Late Caldera-Fill Sequence includes the Mount Pleasant Porphyry and
its associated breccias, and felsic dykes, the Big Scott Mountain
Formation and the Kleef Formation. The ages of the Late Caldera-Fill
rocks are not firmly established; they are most likely Late Devonian but
could range into the Mississippian.
The Mount Pleasant Porphyry is restricted to the Mount Pleasant area
where it occurs as dykes and small, plug-like bodies associated with
magmatic-hydrothermal breccias, as defined by Sillitoe (1985). The dykes
commonly exhibit flow-banding, and crosscutting relationships between
the dykes indicate multiple stages of intrusion. Two types of
hydrothermal breccia are present: an older and more voluminous felsic
phase, and a younger, chloritic phase (Kooiman and others, 1986). The
porphyry, which grades into granitic rocks at depth, and associated
breccias were emplaced at the
The Big Scott Mountain Formation consists of, porphyritic to nearly
aphyric rhyolite, lithic to lithic lapilli-tuff and crystal tuff. Most
of the rhyolites are characterized by pyroxene(?) pseudomorphs. One of
the rhyolite units appears to disconformably overlie the McDougall Brook
Granite. The lithic tuffs contain clasts that were derived from the
Seelys Formation, McDougall Brook Granite, and aphyric rhyolite of
uncertain correlation. Primary layering is discernible in the crystal
tuff and is defined by slight differences in crystal size and abundance.
The Kleef Formation includes redbeds, porphyritic to
glomeroporphyritic basalt and pumiceous, lithic tuff to lithic
lapilli-tuff. Pebble- to cobble-conglomerate contains clasts of the
Scoullar Mountain and Seelys formations, plus the Bailey Rock Rhyolite
and Big Scott Mountain Formation. The basalt is characterized by large
plagioclase phenocrysts (up to 2 cm) and, near the top of the unit, some
plagioclase glomerocrysts (up to several centimetres). The lithic tuffs
are characterized by their reddish brown colour and abundant
Mount Pleasant Deposits
The Mount Pleasant deposits are associated with hydrothermal breccias
and intrusive rocks that cut the Intracaldera Sequence. The various
granite phases and the fluids that produced the mineral deposits were
probably derived by in situ (i.e. no eruption) cooling of peraluminous
anorogenic magma by convective fractionation (diagram),
a term coined by Rice. (1981). Mass balance calculations show that under
the above conditions, small volumes (10-20 km3) of magma with an initial
composition like the Little Mount Pleasant Formation could yield
quantities of metal and fluid capable of producing the Mount Pleasant
The intracaldera host rocks have
been highly brecciated, altered and mineralized in two areas at Mount
Pleasant designated as the North Zone and the Fire Tower Zone (diagram).
In both areas, breccias and associated intrusive rocks form irregular,
roughly vertical, pipe-like complexes that were centers of subvolcanic
intrusive and related hydrothermal activity. The breccias range from
matrix-supported with rounded fragments to clast-supported with mainly
angular fragments. Both fragments and matrix material have been altered
extensively and in many places the fragment protoliths are difficult to
The contact relationships among
and distribution of various granitic phases at Mount Pleasant (diagram)
have been determined from drill cores and exposures in underground workings.
These units, from oldest to youngest, have been designated Granite I,
Granite II and Granite III, referring to fine-grained granite, granite
porphyry and porphyritic granite respectively. All are considered to be part
of the Mount Pleasant Porphyry.
Granites I, II and III are
considered to represent successive cooling stages of one magma body. Granite
I occurs as irregular bodies closely associated with the hydrothermal
breccias. Its contacts with the breccias are commonly gradational and
fragments of Granite I are abundant locally within the breccias. Granite I
is typically fine-grained and equigranular in relatively unaltered
specimens. However, in most areas textural features of Granite I have been
obscured by pervasive chloritic and/or silicic alteration.
Granite II gradationally
underlies Granite I, although in places dyke-like bodies of Granite II have
intruded Granite I and the overlying breccias. Banded porphyry dykes that
crop out at surface are probably derived from Granite II. Granite II varies
from aplitic to porphyritic in texture. Parts of Granite II contain abundant
miarolitic cavities and comb quartz layers. The comb quartz layers consist
of parallel to subparallel layers in which quartz crystals are oriented
approximately perpendicular to the planes of layering. They are one of a
family of unidirectional solidification textures (USTs) that are associated
with fluid saturated and/or undercooled magmas.
Granite III forms a large body
that gradationally underlies Granite II and locally intrudes both Granites I
and II. The contacts are commonly sharp and in many places are marked by
thin (0.5 to 2 cm wide) layers of USTs, mainly K-feldspar, in Granite III.
Granite III varies from fine- to medium-grained and equigranular porphyritic
and pegmatitic. Miarolitic cavities filled with very fine-grained sericite
are locally abundant.
The absolute age of the granitic
rocks at Mount Pleasant, collectively referred to as Mount Pleasant
Porphyry, is uncertain. It has to be younger than the Exocaldera Sequence of
the Piskahegan Group, which is constrained by a U-Pb radiometric age of
363.4 ± 1.8 Ma on the Bailey Rock Rhyolite. K-Ar and Rb-Sr studies indicate
a Late Mississippian age of 340 to 330 Ma. However, a K-Ar date of 361 ± 9
Ma from biotite hornfels in sedimentary breccia that is underlain by Granite
III appears to confirm a Late Devonian-Early Mississippian age.
The mineralization at Mount
Pleasant is granite-related. Tungsten-molybdenum deposits appear to be
related to Granite I; tin deposits are associated mainly with Granite II and
associated with porphyry dykes. Only a few isolated tin zones have been
found within Granite III.
The resource in the Fire Tower Zone prior to mining totalled 22.5 million
tonnes grading 0.21% W, 0.10% Mo and 0.08% Bi (Parish and Tully, 1978);
approximately 11 million tonnes of similar grade material are present in the
North Zone. Included in this resource was a higher-grade deposit in the Fire
Tower Zone containing 9.4 million tonnes grading 0.39% WO3 and 0.20% MoS2
(Kooiman and others, 1986). During the two years of mining this deposit from
1983 to 1985, the Mount Pleasant Mine produced more than 2000 tonnes of
concentrate grading 70% WO3 from about one million tonnes of ore.
The tungsten-molybdenum deposits
are hosted mainly by breccia, Granite I and, to a lesser extent, by
associated country rocks. The deposits consist of mineralized fractures,
quartz veinlets and disseminations in breccia matrix. Wolframite and
molybdenite are the principal ore minerals; minor amounts of bismuth and
bismuthinite are also present. Quartz, topaz, fluorite, arsenopyrite and
loellingite are the principal gangue minerals.
Alteration associated with the
tungsten-molybdenum deposits includes several different types. Intense and
pervasive silicic or greisen-type alteration occurs within and above the
high-grade tungsten-molybdenum zones. This type of alteration is
characterized by the complete or nearly complete replacement of host rocks
by quartz, topaz and fluorite. This alteration grades outward to a less
intense silicic alteration that is limited mainly to narrow selvages on
mineralized fractures and quartz veinlets. Quartz, biotite, chlorite and
minor amounts of topaz are the principal minerals of this alteration stage
that extends laterally up to 100 m beyond the high-grade tungsten-
molybdenum zones. Propylitic alteration consisting of chlorite and sericite
surrounds the silicic alteration and extends for more than 1000 m before
grading into relatively unaltered rock.
Tin-base metal deposits occur as sulphide-rich polymetallic veins and
replacement bodies, which are superimposed on the tungsten-molybdenum
mineralization. Sphalerite, chalcopyrite, arsenopyrite and cassiterite are
the dominant ore minerals and are associated with chlorite, fluorite and a
complex assemblage of sulfides and sulpharsenides, including loellingite,
galena, pyrite, marcasite, molybdenite, tennantite, bornite, bismuthinite,
wittichenite and roquesite.
Most of the potentially economic
tin deposits occur in the North Zone at depth of 200 to 400 m below surface.
They include the Deep Tin Zone, Contact Crest, Contact Flank and
Endogranitic Zone deposits (diagram).The
Deep Tin Zone is a relatively large, irregular deposit that consists of
fracture-controlled and disseminated cassiterite in silicified and
chloritized breccia and Granite I. Other minerals associated with
cassiterite include arsenopyrite, sphalerite, chalcopyrite and galena. The
Contact Crest and Contact Flank deposits occur mainly in breccia or other
associated host rocks at the upper contact or along the sides of Granite II.
The Endogranitic Zone deposit, on the other hand, occurs mainly within
Granite II. In these deposits, cassiterite occurs as finely disseminated
grains and as fine- to medium-sized grains in veins or veinlets and along
fractures. Associated minerals include arsenopyrite, sphalerite,
chalcopyrite, pyrite and pyrrhotite. Chlorite, fluorite, quartz, topaz and
sericite are the main alteration minerals. Crosscutting relationships
indicate that as many as 6 stages of alteration and mineralization may be
present. The total inferred and indicated resources in these North Zone
deposits are 4.8 million tonnes grading 0.82% Sn and 129 g/t In (Sinclair
and others, 2006, their Table 1).
Some of the tin-bearing
polymetallic deposits in the Fire Tower Zone contain significant amounts of
indium, with grades ranging from 50 to 300 g/t In. The indium occurs mainly
as solid solution in sphalerite and, to a lesser extent, in chalcopyrite and
stannite. The total inferred and indicates resources in the Fire Tower Zone
are 0.28 million tonnes grading 0.30% Sn and 207 g/t In (Sinclair and
others, 2006, their Table 1).
Field Trip Stops
STOPS 1-13 are shown here. STOPS
10-13 are shown in greater detail here.
Pumiceous Lithic Tuff, Kleef Formation
Porphyritic Basalt, Kleef Formation
Flow-Banded Rhyolite, Big Scott Mountain Formation
McDougall Brook - Seelys Contact
Little Mount Pleasant Formation and McDougall Brook Granite
Silicified Breccia in McDougall Brook Granite
Little Mount Pleasant Formation
Mount Pleasant North Zone
Mount Pleasant Fire Tower Zone
Cassiterite-Bearing Samples from the Endozone
- Boorman, R.S. and Abbott, D. 1967. Indium in co-existing minerals
from the Mount Pleasant tin deposit. Canadian Mineralogist, v. 9, p.
- Dagger, G.W. 1972. Genesis of the Mount Pleasant
tungsten-molybdenum-bismuth deposit, New Brunswick, Canada. Institute of
Mining and Metallurgy Transactions, v. 81, section B, p. 73-102.
- Davis, W.J. and Williams-Jones, A.E. 1985. A fluid inclusion study
of the porphyry-greisen, tungsten-molybdenum deposit at Mount Pleasant,
New Brunswick. Mineralium Deposita, v. 20, no. 94, p. 94-101.
- Hosking, K.F.G. 1963. Geology, mineralogy and paragenesis of the
Mount Pleasant tin deposits. Canadian Mining Journal, v. 84, no. 4, p.
- Hunt, P.A. and Roddick, J.C. 1990. A compilation of K-Ar ages,
Report 19. In Radiogenic age and isotopic studies. Report 3, Geological
Survey of Canada, Paper 89-2, p. 153-190.
- King, M.S. and Barr, S.M. 2003. Southern New Brunswick potential
fields project, Part I; Implications for granite-related gold
mineralization in the Clarence Stream area. New Brunswick Department of
Natural Resources and Energy, Minerals, Policy and Planning Division,
Open File 2003-6, 49 p.
- Kirkham, R.V. and Sinclair, W.D. 1988. Comb quartz layers in felsic
intrusions and their relationship to porphyry deposits. In R.P. Taylor
and D.F. Strong, eds., Recent advances in the geology of granite-related
mineral deposits. The Canadian Institute of Mining and Metallurgy,
Special Volume 39, p. 50-71.
- Kooiman, G.J.A., McLeod, M.J. and Sinclair, W.D. 1986. Porphyry
tungsten-molybdenum orebodies, polymetallic veins and replacement
bodies, and tin-bearing greisen zones in the Fire Tower Zone, Mount
Pleasant, New Brunswick. Economic Geology, v. 81, p. 1356-1373.
- McCutcheon, S.R. 1987. Mount Pleasant caldera project. In S.A.
Abbott, ed., Twelfth Annual Review of Activities. New Brunswick
Department of Natural Resources and Energy, Minerals and Energy
Division, Information Circular 87-2, p. 47-50.
- McCutcheon, S.R. 1990a. The Late Devonian Mount Pleasant caldera
complex: stratigraphy, mineralogy, geochemistry and the geologic setting
of a Sn-W deposit in southwestern New Brunswick. Unpublished Ph.D.
thesis, Dalhousie University, Halifax, 609 p.
- McCutcheon, S.R. 1990b. The Mount Pleasant caldera: geological
setting of associated tungsten-molybdenum and tin deposits. In D.R.
Boyle, ed., Mineral deposits of New Brunswick and Nova Scotia [Field
Trip 2], 8th IAGOD Symposium Field Trip Guidebook, Geological Survey of
Canada, Open File 2157, p. 73-77.
- McCutcheon, S.R., Anderson, H.E. and Robinson, P.T. 1997.
Stratigraphy and eruptive history of the Late Devonian Mount Pleasant
caldera complex, Canadian Appalachians. Geological Magazine, v. 134, p.
- McGregor, D.C. and McCutcheon, S.R. 1988. Implications of spore
evidence for Late Devonian age of Piskahegan Group, southwestern New
Brunswick. Canadian Journal of Earth Sciences, v. 25, p. 1349-1364.
- Parrish, I.S. 1977. Mineral catalog for the Mount Pleasant deposit
of Brunswick Tin Mines. Canadian Mineralogist, v. 15, p. 121-126.
- Parrish, I.S. and Tully, J.V. 1978. Porphyry tungsten zones at Mt.
Pleasant, N.B. The Canadian Institute of Mining and Metallurgy Bulletin,
v.71, no. 794, p. 93-100.
- Pearce, G. 1989. NovaGold offers $12 million for Mount Pleasant tin
mine. The Northern Miner, v. 75, no. 35, p. A1-A2.
- Petruk, W. 1973a. The tungsten-molybdenum-bismuth deposit of
Brunswick Tin Mines Limited; Its mode of occurrence, mineralogy, and
amenability to mineral benefaction. The Canadian Institute of Mining and
Metallurgy Bulletin, v. 66, no. 732, p. 113-130.
- Petruk, W. 1973b. Tin sulfides from the deposit of Brunswick Tin
Mines Ltd. Canadian Mineralogist, v. 12, p. 46-54.
- Pouliot, G., Barondeau, B., Sauve, P. and Davis, M. 1978.
Distribution of alteration minerals and metals in the Fire Tower zone at
Brunswick Tin Mines Ltd., Mount Pleasant area, New Brunswick. Canadian
Mineralogist, v. 16, p. 223-237.
- Rice, A. 1981. Convective fractionation: a mechanism to provide
cryptic zoning (macrosegregation), layering, crescumulates, banded tuffs
and explosive volcanism in igneous processes. Journal of Geophysical
Research, v. 86B, p. 405-417.
- Ruitenberg, A.A. 1963. Tin mineralization and associated rock
alteration at Mount Pleasant, Charlotte County, New Brunswick.
Unpublished M. Sc. thesis, University of New Brunswick, Fredericton, 172
- Ruitenberg, A.A. 1967. Stratigraphy, structure and metallization,
Piskahegan-Rolling Dam area (Northern Appalachians, New Brunswick,
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Unidirectional solidification textures and their significance in
determining relative ages of intrusions at the Henderson Mine,
Colourado. Geology, v. 10, p. 293-297.
- Sillitoe, R.H. 1985. Ore-related breccias in volcanoplutonic arcs.
Economic Geology, v. 80, p. 1467-1514.
- Sinclair, W.D. 1994. Tungsten-molybdenum and tin deposits at Mount
Pleasant, New Brunswick, Canada: products of ore-fluid evolution in a
highly fractionated granitic system. In R. Seltmann, H. Kämpf, and R.
Möller, eds., Metallogeny of collisional orogens, Czech Geological
Survey, Prague, p. 410-417.
- Sinclair, W.D. and Kooiman, G.J.A. 1990. The Mount Pleasant
tungsten-molybdenum and tin deposits. In D.R. Boyle, ed., Mineral
deposits of New Brunswick and Nova Scotia [Field Trip 2], 8th IAGOD
Symposium Field Trip Guidebook, Geological Survey of Canada, Open File
2157, p. 78-87.
- Sinclair, W.D., Kooiman, G.J.A. and Martin, D.A. 1988. Geological
setting of granites and related tin deposits in the North Zone, Mount
Pleasant, New Brunswick. In Current Research, Part B, Geological Survey
of Canada, Paper 88-1B, p. 201-208.
- Sinclair, W.D., Kooiman, G.J.A., Martin, D.A. and Kjarsgaard, I.M.
2006. Geology, geochemistry and mineralogy of indium resources at Mount
Pleasant, New Brunswick. Ore Geology Reviews Vol. 28, p. 123-145.
- Sutherland, J.K. and Boorman, R.S. 1969. A new occurrence of
roquesite at Mount Pleasant, New Brunswick. American Mineralogist, v.
54, p. 1202-1203.
- Tucker, R.D., Bradley, D.C., ver Straeten, C.A., Harris, A.G.,
Ebert, J.R. and McCutcheon, S.R. 1998. New U-Pb zircon ages and the
duration and division of Devonian time. Earth and Planetary Science
Letters, v. 158, p. 175-186.
- van de Poll, H.W. 1967. Carboniferous volcanic and sedimentary rocks
of the Mount Pleasant area, New Brunswick. New Brunswick Department of
Natural Resources, Mineral Resources Branch, Report of Investigation 3,
- Williams, D.A. 1978. Fredericton (21G), Bouger gravity map, scale
1:250,000. New Brunswick Department of Natural Resources, Minerals
Division, Map Plate 78-42b.
- Yang, X, Lentz, D. R. and McCutcheon, S.R. 2003. Petrochemical
evolution of subvolcanic granitoid intrusions within the Late Devonian
Mount Pleasant Caldera, southwestern New Brunswick, Canada: comparison
of Au versus Sn-W-Mo-polymetallic mineralization systems. Atlantic
Geology, 39, p. 97-121.