OQ' 



32 THE PENOKEE IKONBEAIilNG SERIES. 



SECTION II.— ORIGIN OF THE UPPER SLATE ROCKS. 



From wliat has preceded, it is evident that the association of the mica- 

 schists and mica7slates of the Upper-skite meniljer with the graywackes 

 and graywacke-slates is of a most intimate nature, both as to composition 

 and occurrence. The same set of minerals occur in both classes of rock; 

 they are closely interstratified witJi each other. There is an identical 

 change in each class in mineral composition in passing from east to west, 

 and in an}" one region the change occurs simultaneously in both classes. 

 Finally, there is every gradation between them, and the placing of many 

 specimens in one class rather than in the other is somewhat arbitrarj-. 

 It becomes, then, probable that the original condition of the graywackes 

 and gray wacke-slates and the mica-schists and mica-slates must in the main 

 have been the same. As some of the graywackes are completely frag- 

 mental, others somewhat crystalline, ( )thers still more crystalline, thus grad- 

 ing into the crystalline mica-schists and mica-slates, we will begin with 

 those which are nearest their original condition and trace the processes step 

 by step in which the original rock has heen clianged into a fully crystal- 

 line mica-schist. 



A general notion of the graywackes and graywacke-slates has already 

 been given. Before proceeding to ti-ace tliese processes of alteration it will 

 be necessary to describe in more detail tlie graywackes wliich are near 

 their original condition. 



Macroscopically the graywackes vary from tolerably coarse grained 

 to a])hanitic. Many of them are apparently completely massive in hand 

 s))ecimen, and such break with conclioidal fracture, althougli even tliese 

 in exposure show more or less of a cleavage along the bedding planes. 

 Specimens of the graywacke-slates show a decided tendency to cleave 

 })arallel to the bedding. In color these rocks vary from light gray or light 

 green, through various shades of gray and green, to almost black. Under 

 tlie microscope the only tKfference between the graywackes and graywacke- 

 sUxtes is one of fineness of grain. The least altered vai'ieties of them may 

 be divided into two classes, one being composed mostly of tolerably large 

 well rounded particles of quartz and feldsj)ai- (PI. xxxii), and the t)ther 



