72 J. A. Dresser — Study in the Metamorphic Rocks 



pora and Ptilodictya are mentioned, but no more definite 

 description seems to have been yet published. 



Quartzite. — With the increase of quartz, the dolomite 

 passes into a calcareous sandstone and eventually to a some- 

 what impure quartzite. This consists of rounded quartz 

 grains, a few grains of feldspar, and where the rock is much 

 deformed by pressure, or else where the original composition 

 was slightly different, shreds of sericite are developed along 

 the planes of foliation. The chief cementing material in typi- 

 cal parts has been silica, and the enlarged grains thus assume 

 irregular interlocking forms. 



A few rather large grains of feldspar, either orthoclase or 

 microcline, are generally found in the thin section as well as 

 occasional ones of a.rhombohedral carbonate, probably dolomite. 

 This rock is commonly interbedded with black mica schist in 

 an intricate manner, apparently indicating transitional phases 

 between the two classes of rocks. 



Amygdaloidal Trap. — The volcanic nature of this rock, 

 which was formerly regarded as an argillite or was more fre- 

 quently designated as a chloritic schist, has been preliminarily 

 noted* by the writer, but opportunity has not yet been found 

 to com|)lete a petrographical examination of it. It is a bluish 

 or greenish gray rock, in some places massive, and is often 

 highly schistose. It contains large numbers of vein-like masses 

 of quartz and some of calcite, is amygdaloidal in many 

 parts, and frequently contains nodular masses three or four 

 inches in diameter which are composed chiefly of rudely con- 

 centric layers of quartz and epidote. 



By the aid of the microscope, crystals of primary plagioclase 

 can be seen whose arrangement is suggestive of the structure 

 of diabase, but as the interstitial material is wholly secondary, 

 chlorite, iron ore, leucoxene, etc., further evidence is desirable 

 in order to determine its precise classification. It appears to 

 have been somewhat variable in its original character, as 

 fibrous hornblende occurs in considerable amount at a point 

 about three miles west of the St. Francis river, but it may be 

 generically classed as a basic volcanic of the andesite or diabase 

 class. It has been extremely altered. 



Structure. 



The contacts of the various sedimentary rocks with one 

 another are, as has been already stated, of the nature of transi- 

 tions from one to another of the different types. The graphi- 

 tic limestone passes upwards into mica schist by an almost 

 insensible gradation, as can be well seen along either bank of 



*The Ottawa Naturalist, January, 1901 ; this Jour., July, 1902. 



