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MUSEUM BULLETIN NO. 27. 
that the igneous rocks of the belt are principally post-Ordovician in 
age, and were probably intruded in pre-Devonian time. While the rocks 
comprising this igneous complex are, for the most part basic, they 
actually range in composition from the most basic to the most acid 
types; the series may be said to include dunite, peridotite, pyroxenite, 
gabbro, porphyrite, hornblende granite, and aplite; although in the 
field there is as a rule no sharp line of demarcation between any two of 
these owing to the occurrence of intermediate types. Dresser believes 
that the whole complex presents a gradual transition from the composition 
of a dunite to that of a granite. These and other considerations have led 
to the view that the various rock types have all been produced by differ- 
entiation from a single magmatic intrusion. 
The rocks of the serpentine belt do not form a continuous unbroken 
band along their strike to the northeast, but appear as a series of isolated 
stock-like masses, which vary considerably, both in length and width. 
In the Black Lake-Thetford area, they outcrop in a northeasterly direc- 
tion for a distance of about 10 miles and roughly form two parallel 
bands about 2 miles apart, each of which is from 2 to 3 miles wide. 
Black lake is situated at about the centre of the northern band, on its 
northwestern margin. 
The two bands may be regarded as a batholith, or possibly a thick 
laccolith, 1 although actually they present the appearance of many 
isolated stocks, which may be connected with one another at depth. 
It is found that the different rock varieties are arranged in the order of 
decreasing density, or basicity, from the centre outwards. Thus in the 
ideal case and where there has been sufficient erosion, peridotite of the 
dunite type is exposed at the central part of such a stock, and as this is 
traced outward the rock is found to become progressively more and more 
acid, passing by gradual transitions through pyroxenite, gabbro, diabase, 
and porphyrite until at the margins of the mass it may be a normal 
hornblende granite. These different rock types are rarely sharply 
distinguished from one another, even the granite in some places having 
been observed as a differentiation product. More frequently, however, 
the granite, together with aplite, has been injected a little later than the 
main intrusion, and it then appears in the form of dykes, sills, and 
intrusive sheets cutting the other more basic types (Plate III). 
It should be stated that in Black Lake area erosion has truncated 
the stocks or the foliated laccolith, to such an extent that the more basic 
central or lower phases, especially peridotite, are now exposed almost 
to the exclusion of the marginal differentiates of more add composition, 
1 Harvie, in a private communication, has informed the writers that in Garthby township, the serpentine 
has the form of a folded iaccolithic sheet lying between quartzites beneath and slates above. See Sum. 
Kept., 1916. 
