64 Adams—Oontribution to our knowledge of the Laurentian. 
shearing accompanied by the development of new minerals, 
which takes place under other conditions and probably nearer 
the surface. 
The third class of rocks referred to as forming part of the 
Laurentian consists of crystalline limestones, quartzites and a 
set of peculiar gneisses, all intimately associated and usually 
interbanded. The limestones are thoroughly erystalline, more 
or less pure granular marbles, which although presenting many 
features of a special interest need not here be further referred 
to. They are in all respects similar to those described by 
various writers in other Laurentian districts.* The quartzites 
which have been examined are entirely crystalline, nothing has 
been detected in them which distinetly proves them to be of 
clastic origin. Professor Rosenbusch, however, believes that 
the specimens from one locality present structures which indi- 
eate that the rock was originally a sandstone. 
The gneisses which are as a general rule intimately asso- 
ciated with the limestones and quartzites are quite different 
from those of the second class just described. They are almost 
always highly garnetiferous and frequently consist essentially 
of garnet and sillimanite. Quartz and orthoclase are present 
in subordinate amount. Some of them contain pyroxene, 
scapolite and other minerals. A very rusty weathering gneiss, 
containing in some cases nearly five per cent of pyrite, is asso- 
ciated in a particularly intimate manner with the limestones, 
often in very thin bands showing wonderful contortions pre- 
cisely like those described by Dr. Kemp in the Adirondack 
region,t and by other writers elsewhere in the Laurentian of 
Canada. | 
These gneisses show no granulated structure, the minerals 
constituting them have crystallized under the influence of the 
pressure which has granulated the gneisses of class 2 and are 
_ not in any marked manner deformed by it. Furthermore 
these rocks are generally well banded; this structure being fre- 
quently much more pronounced than the foliation, and graphite, 
which does not occur in the igneous granulated gneises of 
class 2, is very frequently present and is often abundant. As 
localities where these rocks are well seen, the northern part of 
the township of Rawdon, the township of Cathcart and the 
district about St. Jean de Matha may be instanced. 
When any granite or granitic rock is for long periods ex- 
posed to a process of gradual decay there results finally a mass 
of kaolin, often. mixed with more or less chloritic material, 
* Logan, Geology of Canada, p. 24, etc.; Hunt, Laurentian Limestones of North 
America, Rep. Geol. Survey of Canada, 1863-66; Kemp, Bull. Geol. Soc. of 
America, 1895. 
t Loe. cit. 
| 
