296 Canadian Record of Science. 



green rim was frequently observed. Having prepared 

 thin sections and examined more carefully the nature of 

 these zones, attention was drawn to them in a short 

 paper. 1 They have also been studied by a number of 

 other investigators. 2 The examination of a large number 

 of additional hand-specimens from this area has, how- 

 ever, brought to light many additional facts concerning 

 this remarkable phenomenon. 



The most massive variety of anorthosite in the whole 

 area is found on the east shore of Lake St. John, one to 

 two miles south of the head of the Saguenay Biver, where 

 it forms large exposures. 



Although the same irregularity in the size of grain 

 as well as in the proportion of the constituents, which 

 so often presents itself in gabbros and other basic 

 rocks, appears in many places, yet nothing like banding 

 in the rocks could be discovered. Distinct sets of cracks 

 cutting the anorthosite cause it to split up into small 

 cubic blocks, as in the case with granite and other 

 plutonic rocks. 



On the examination of thin sections under the micro- 

 scope, olivine and feldspar are seen with the above-men- 

 tioned zones around the former. Some small grains of 

 horneblende, ilmenite and pyrite are likewise generally 

 present. Like the olivine, the plagioclase is quite fresh 

 and contains no products of decomposition. It has a 

 specific gravity of 2.70 to 2.71. The maximum extinction 

 was determined in many thin sections and showed 32J° 

 on either side of the twinning line. The mineral is, there- 

 fore, bytonite. It is almost black, being rilled with the 



i Adams, Notes on zones of certain silicates occurring about the olivine in anortho- 

 site from the Saguenay District — Am. Nat., Nov., 1885. 



2 J. G Bonney, Troktolite in Aberdeenshire— Geol. Mag., Oct., 18S5. J. H. 

 Hatch, Notes on the Petrographical characters of some rocks collected in Madagascar, 

 Q.J.G.S., May, 1889. J. W. Judd, Chemical Changes in Rocks under Mechanical 

 Stresses. — Journ. Chem. Soc, London, May, 1890. A. E. Tornebohm, Uber die wich- 

 tigeren Gabbro— unci Diabas-Gesteine Schwedens. Neues Jahrb. fiir Min , etc., 1S77, 

 383. G H. Williams, Peridotites of the Cortlandt Series.— Am. Jour, of Sc, Jan, 1886 



