﻿454 Canadian Record of Science. 



This was formerly supposed to indicate the lines of an almost obliter- 

 ated bedding, in a rock of aqueous origin, but recently it has been 

 proved that igneous rocks, under great pressure, develop the same 

 parallel arrangement of their mineral constituents, so now it can no 

 longer be the test to decide the origin of a rock. 



Owing to the variety and complexity of the gneisses, they have been 

 divided into three classes, namely : gneisses of igneous, aqueous, and 

 of doubtful origin. 



Gneisses of igneous origin contain a large amount of orthoclase fel- 

 spar, with quartz, mica or hornblende in smaller quantities, and many 

 acessory minerals. The three principal varieties are the augen, gran- 

 ulated and leaf gneisses, and they are connected to one another by 

 transitional types. 



The first variety occurs in the township of Brandon, associated with 

 stratified rock of the Grenville series. It is composed essentially of 

 quartz, orthoclase felspar in large amount, and hornblende in a 

 fine-grained, ground mass of the two latter minerals. Under the 

 microscope the orthoclase grains show strain shadows, and are very 

 irregular in shape, having an angular outline, which is due to the 

 constant breaking away of particles from the edges. The quartz 

 appears rolled out in long lines, or as curved grains partly surrounding 

 the orthoclase cores. Koth minerals have an vineven extinction, but 

 the latter only to a very slight degree. This rock was originally a 

 coarse-grained granite, and received its present structure from pressure. 



The granulated is the next type. A typical representative of this is 

 the Fundamental Gneiss of Logan, Trembling Mountain being the great 

 development of it. When fresh it is pale red in color, weathering to 

 a grayish ; it is uniform in appearance, and slightly foliated. There 

 are two kinds of orthoclase, a fibrous, showing strain shadows, while the 

 other does 'not, being derived from the crushed augen, for when the 

 pressure is removed the strain shadows disappear. In chemical com- 

 position it is a granite, originally of the hornblende variety. The leaf 

 gneiss represents the third and extreme type. It is found near St, 

 Jerome, and is composed of orthoclase in large amount, having no 

 distinct strain shadows, and quartz. The latter is in very thin layers, 

 and when the rock is broken parallel to the line of foliation it seems to 

 be spread over the orthoclase as in leaves, hence its name. "The 

 structure suggests a completely granulated rock, in which the granula- 

 tion has perhaps been effected, in part at least, by crystallization." 



The gneisses of aqueous origii) form the next class — are so determined 

 chiefly from the fact that they have a chemical composition identical 

 with shales or other sediments, but quite different from that of any 

 igneous rock. These gneisses, in addition to the essential constituents 

 of the igneous class, contain garnet, sillimanite and graphite. They 

 are divided according to their mode of weathering— one variety crum- 



