506 Notices of Memoirs — Anorthosite Rocks of Canada. 



presented in the form of a preliminary map, accompanied by descrip- 

 tions of routes and passes, and remarks on the main orographic features 

 of the range. 1 



IV. — The Anorthosite Rocks op Canada. By Frank D. Adams, 

 Geological Survey of Canada. 



THIS series of rocks has also been called the Upper Laurentian or 

 JSTorian series. The name anorthosite is perhaps preferable, as it 

 refers to their distinguishing characteristic as compared with the ortho- 

 clase rocks of the Lower Laurentian, viz. the predominance in them of 

 plagioclase or anorthose felspar. These rocks form detached areas in 

 the great Laurentian districts, and bear a strong resemblance in part 

 to the gabbros and gabbro-diorites of Scandinavia, and in part to the 

 labradorite rock of the same country. It is, however, by no means 

 certain that the rocks of the two countries are of the same age. At 

 least nine of these areas are now known to exist in Canada, and there 

 is also one in the State of New York. In addition to plagioclase, 

 which generally predominates largely, these rocks contain rhombic and 

 monoclinic pyroxenes (including augite, diallage, hypersthene, and 

 probably enstatite), olivine, magnesia, mica, spinel (including both 

 pleonaste and picotite), garnet, iron-ores, pyrite, and apatite. Ortho- 

 clase is seldom or never found, except in veins cutting the anorthosite. 

 The hornblende, mica, and pyroxenes are intimately associated and 

 often intergrown, all of them sometimes being found in the same thin 

 section. Garnet occurs sparingly, and generally near the contact of 

 the anorthosite with the gneiss. When the olivine comes against 

 plagioclase, it is always bounded by a double concentric zone, the outer 

 zone consisting of hornblende, and the inner, or that next to the 

 olivine, consisting of a pyroxene. While the iron-ores associated with 

 the Lower Laurentian gneisses are generally free from titanium, those 

 associated with the anorthosite rocks are always highly titaniferous ; a 

 fact which makes the study of these rocks a matter of considerable 

 economic interest. The anorthosite varies a good deal in composition, 

 some areas, for instance, being rich in olivine, while others are destitute 

 of that mineral, and different portions of even the same area often 

 showing wide differences in this respect. The rock also shows a good 

 deal of variation in structure. It is rarely quite massive, frequently 

 well foliated, but usually consists of a rather coarsely crystalline ground- 

 mass, through which are scattered irregular strings and masses com- 

 posed of iron-ore, bisilicates, and mica, as well as larger porphyritic 

 crystals of plagioclase. Even when it is tolerably constant in com- 

 position, there is generally a great variation in size of grain, coarse and 

 fine alternating in rude bands or rounded masses. In the case of some 

 of the areas there can be but little doubt that the anorthosite is 

 eruptive, in others, however, it seems to be interstratified with the 

 Laurentian gneiss, and in one of them to merge imperceptibly into it. 

 The original relations of the rocks are, of course, much obscured by the 

 effects of subsequent heat and pressure. The evidence at present, how- 

 ever, seems to indicate that these anorthosites are the result of some 

 kind of extravasation, which in those early times corresponded to what 

 in modern times we call volcanic eruption. 



1 See Eeports and Maps Geological Survey of Canada. 



