﻿Booh Notices. 455 



bling to a rusty sand, caused by the iron pyrites it contains, and the 

 other variety, being free from that mineral, not doing so. 



An example of the former is found in a garnetiferous sillimanite 

 gneiss occurring near St. Jean de Matha, interbanded with quartdte. 

 It is dark gray on a fresh surface, and contains, in addition to tiie two 

 minerals which give it its name, quartz, orthoclase, pyrite and 

 graphite ; the long crystals lie in one direction, and so give an indis- 

 tinct foliation to the rock. It is considered probable that the pyrite 

 was deposited by the agency of the carbon, in the form of graphite 

 before the rock had crystallized. Under the microscope no granulation 

 is seen in the minerals which were crystallized in situ, but the rock 

 has undergone some pressure since, as the quartz and sillimanite show 

 uneven extinction. The garnet is isotropic, and holds inclusions of 

 other minerals. The rock of the second variety, which contains the 

 same constituents as the above, with the exception of the pyrite, is 

 found north-west of St. Jean de Matha, occurring in horizontal beds. 

 The rock is red in color, and contains a large amount of garnet. The 

 microscopical characteristics are the same as the last. The chemical 

 composition of this class is that of a shale, the percentages of alumina 

 being high, while those of potash and soda are correspondingly low. 



The crystalline limestones are important, not only economically, but 

 also on account of their being found interbanded with gneisses ; they 

 form a valuable aid in the solving of the stratigraphical problems. 

 In some places they are mixed with gneissic material, and so form 

 calcareous gneisses. 



In the eastern part of the map they lie in horizontal beds, some 

 having a considerable thickness ; near the Morin anorthosite, several 

 outcrops appear, being parts of the same band, which has been repeat- 

 edly brought to the surface by folding, as illustrated in the cross 

 sect^ions. Beds of all sizes occur in every township in the south 

 and west, some being quite pure, while others contain a great 

 many minerals, such as graphite, tourmaline, serpentine, etc. With 

 reference to the origin of the last named mineral, it was found that the 

 core of the lump was pyroxene, so conclusively proving that the ser- 

 pentine of the Laurentian crystalline limestones, here at least, is 

 formed by the alteration of pyroxene. 



Examined microscopically, the limestone is crystallized in its usual 

 characteristic rhombohedral form ; in some cases part of the crystal is 

 very clear, while the remainder is turbid, which latter character oflfers 

 an interesting field for future investigation. 



The third class of gneisses, or those of doubtful origin, are numer- 

 ous and occur in many parts of this area. The varieties include 

 quartz orthoclase biotite gneiss, with and without garnet, garnetifer- 

 ous hornblende gneiss, granulite, and pyroxene and amphibolite granu- 

 lites. These, as in the case of the former classes, have been submitted 



