ALGONKIAN ROOKS OF STUEGEON ElVEE TONGUE. 479 



The purple arkoses differ from the puik ones just described in contain- 

 ing chlorite and hornblende, and in addition some apparently newly formed 

 feldspar, notably a feldspar with the microcline twinning. As a rule, these 

 rocks are more feldspathic than the matrices of the conglomerates, and they 

 contain much less quartz. The larger grains of both quartz and feldspar 

 are corroded as if partially dissolved. They have lost their smooth, rounded 

 contours of sand grains, and now possess irregular jagged ones, which, 

 however, are not due to secondary enlargements. 



The characteristic components of these rocks are the chlorite and the 

 hornblende. The former mineral is present in plates intermingled with 

 grains of epidote, while the hornblende is in dark-green or light-green 

 plates, and in acicular or columnar crystals that are idiomorphic in cross 

 section. The crystals are distributed indiscriminately through the rocks, 

 with their longer axes lying in all azimuths. They were evidently formed 

 after the squeezing that made the rock schistose. The plates, moreover, 

 include within themselves such great numbers of the other components of 

 the rock that their parts often appear to be independent. Under crossed 

 nicols, however, many of these apparently independent plates are discov- 

 ered to polarize together. No evidence is present in any of the sections as 

 to the source of the material that gave rise to the hornblende. The fact, 

 however, that all of the hornblendic rocks are banded, that some layers are 

 rich in amphibole while others are completely devoid of this mineral, sug- 

 gests the notion that the hornblendic schistose arkoses consist partly of 

 sedimentary and partly of tuffaceous materials. As we shall see later, this 

 origin is ascribed with more confidence to some very peculiar rocks to be 

 discussed later. 



Crushing effects are noticed in some of the hornblendic arkoses, but 

 their present condition appears to be due more to chemical changes pro- 

 duced in them than by mechanical action. The chemical changes were no 

 doubt superinduced by the mashing, but this can only be inferred from the 

 fact that they are more pronounced in the schistose phases of the rocks than 

 in those phases in which the schistosity is poorly developed. 



THE DOLOMITE FORMATION. 



The dolomite formation comprises, as has been stated, both dolomitic 

 limestones and calcareous slates, and occasionally quartzites, sandstones, 



