988 THE GEOLOGY OF MINNESOTA. 



[The oldest known rocks. 



by the Canadian geologists, into Lower and Upper, according to recent researches 

 by Adams, seems to be invalidated, since the Upper Laurentian is found to consist 

 principally of intrusives, such as anorthosyte and other gabbros. However, Logan's 

 separation of the Lower Laurentian into Grenville series and Ottawa gneiss is a 

 distinction which in literature has been maintained. The supposed lower portion, 

 the Ottawa gneiss, is mainly igneous, according to Adams, and the Grenville series 

 is mainly of sedimentary origin. 



In the Rainy Lake region Lawson, in the same way, considers the " Laurentian'' 

 which invades the Coutchiching and Keewatin, as igneous, and as belonging nor- 

 mally below the Ontarian or sedimentary series. If, however, a careful examination 

 be made into the nature and structure of the Keewatin of Dr. Lawson, it will be 

 found, notwithstanding the apparent parallelism of the succession of the Ottawa 

 region with that of Rainy lake, there is a notable disagreement in structure and 

 succession. As described by Lawson the Keewatin consists of two parts, viz., at the 

 bottom are basic rocks, diabase and hornblende schists. These are followed upward 

 by more acid rocks, but without a distinct line of separation. The whole series is 

 supposed by him to have been primarily of volcanic origin, the nature of the ejecta 

 having changed from basic to acid. This volcanic series he believes is nonconform- 

 able upon the Coutchiching which consists of mica schists derived plainly from clastic 

 materials by recrystallization, having a thickness of 22,000 to 28,000 feet. As evidence 

 of this nonconformity he refers to a conglomerate which is nonconformable upon 

 the Coutchiching and lies, as he supposed, at the bottom of the Keewatin. This 

 nonconformity we never could find in Minnesota, but everywhere the Keewatin rocks 

 pass gradually into mica schists. It is therefore a matter of considerable interest 

 that Mr. A. P. Coleman has recently shown that the very conglomerate to which 

 Lawson appealed contains much Keewatin material of older date, and that hence the 

 horizon of the conglomerate is high up in the Keewatin itself.* It is to be noticed 

 therefore, by the removal of this nonconformity above the mica schists, that the 

 lowest known portion of the Keewatin consists, both in the Rainy Lake region and 

 in Minnesota, of what in the Lake Superior region generally has been called green- 

 stones. 



According to Lawson, and also according to all observations made on this 

 horizon in Minnesota, these greenstones are overlain by a series of volcanic detrital 

 rocks of great thickness, and it is these volcanic detrital rocks, with their variations 

 to true clastic sediments, which, in Minnesota, are extensively converted into gneisses 

 and mica schists by metamorphism. They are parts of the Lower Keewatin. 

 Putting these facts together, it is necessary to come to the conclusion that the mica 



*Re.port of the. Bureau oj Mines (Ontario) for 1897. Toronto, 1898, Part II, p. 153. 



