MINERALOGY AND PETROLOGY. 989 



The oldest known rocks.] 



schists are later than the greenstones rather than earlier, structurally as well as 

 petrographically, i. e., that some portions of the Keewatin are metamorphosed. If 

 the Rainy Lake succession can be adjusted with that of Minnesota, the order will be 

 something as follows: 



At the bottom is a greenstone, sometimes massive and igneous, but frequently 

 passing into an agglomeratic condition, with which are associated various other 

 fragmental strata, mainly "greenstone conglomerates," jaspilyte, greenwackes, but 

 also slates and fine graywackes. 



The first appearance of acid rock is in the form of a quartz-porphyry (No. 2229) 

 immediately following or in the midst of the fragmental part of this greenstone. 



In some places the clastic portion associated with the quartz-poryhyry reaches 

 a great thickness (perhaps 20,000 feet), and by a gradual increase of acid materials 

 through erosion of the quartz-porphyry, as well as by chemical precipitation from 

 the ocean, it acquired a decidedly acid character. . A metamorphic revolution accom- 

 panied by the production of granite and other crystalline rocks terminated this part 

 of the Keewatin. 



There accumulated then a great conglomerate which we have taken as the base 

 of the Upper Keewatin, and which is presumably the equivalent of that described 

 by Lawson and Coleman in the Rainy Lake district. This conglomerate lies some- 

 times on the earlier granite, and sometimes on the greenstones. Although it would 

 be necessary to infer that it also lies on the strata intervening between the granite 

 and the greenstones, whether they were metamorphosed or not, we have rarely seen 

 it so superimposed, but from the facts stated by Lawson it appears that at Rainy 

 lake it lies on the mica schists (Coutchiching) produced by the metamorphism of 

 those strata. In Minnesota this conglomerate has been called at one locality Stuntz 

 conglomerate, and at another Ogishke conglomerate, before it was sufficiently shown 

 that it is at the same horizon at both places. 



This conglomerate is Variously mingled with greenstone debris where it lies 

 upon the above mentioned greenstones of the Lower Keewatin, and by the gradual 

 loss of the conglomeratic composition it becomes a clastic greenstone, and as such it 

 acquires great thickness. It appears to have been augmented also by volcanic 

 materials and by further chemical precipitation. In other places the conglomeratic 

 composition is followed by great thicknesses of graywackes and slates. This epoch of 

 the Keewatin was closed by another grand onset of metamorphism and igneous 

 intrusion, which was followed by the Animikie. 



Lastly, what the exact manner of transition was to the Animikie is not known, 

 but the Animikie seems to have a basal conglomerate along the southern slopes of 

 the Giant's range; volcanic action was resumed and was widespread at the opening 

 of the Animikie. 



