GLACIAL ORIGIN OF HURONIAN ROCKS 



461 



sibly into typical arkoses. They are usually very massive, fine to 

 medium grained, and not unlike light-colored granite in appearance. 

 It is often very difficult to determine their structure as there are 

 seldom well-marked bedding planes. There are however instances 

 in which the bedding is indicated by variations in the size and relative 

 proportions of the various grains and other cases in which it is indi- 

 cated by horizontal jointing. The rock is very largely composed of 

 quartz and feldspar. Sericite and kaolin are prominent in light- 

 colored varieties and chlorite in the darker. Titanite and iron 



Fig. 2. — Huronian conglomerate, Temagami, Ont. 

 foot long. 



Steel scale (in book) is one 



ores are usually present in small quantity. The feldspar and quartz 

 grains are often well rounded but quite as frequently angular or sub- 

 angular. 



Closely allied to the shales and arkoses are the graywackes. The 

 chief recognizable constituents in these are feldspar, quartz, a dark 

 chlorite, and a pale-colored mica. Less abundant are small particles 

 of iron ore and epidote, while pyroxene and amphibole are rare. 

 With the minerals are angular and rounded rock particles of various 

 sizes. Rock of this type in some instances is found in massive beds 

 of uniform character, very fine grained and of gray to greenish color. 

 Similar material forms the matrix of much of the bowlder conglomerate. 



