10 THE GEOLOGY OP MINNESOTA. 



[Metamorphism of the fragmental rocks 



to a great series of gneisses and of mica schists, which in some of the annual reports 

 has been designated by Lawson's term Coutchiching. They lie nearest the axes of 

 upward flexure, and hence also nearest the large areas of granite. The crystallization 

 produced in the Archean strata by this profound earth-movement is not wholly due 

 to the act of contacting on the igneous rock, for it extends sometimes for a score or 

 more of miles from the granitic area. It must be attributed rather to the general 

 influence of a regional and uniform rise in the temperature of the earth's crust along 

 those belts where this effect is seen, accompanied by an intensified action of heated 

 water and vapor. It is apparent, therefore, and this is in accord with observation, 

 that the mica schist and the gneiss may have resulted at any horizon in the early 

 fragmentals, and that such rocks cannot from their mineral composition be taken to 

 be older than other Keewatin rocks that do not show this metamorphism. In other 

 words, the stratigraphic value of such a term as Coutchiching is nil, when it is applied 

 to the general chronological scheme. It can only express a greater nearness to an 

 accidental and local centre of metamorphism or to an igneous protrusion. 



It is very true that Mr. Lawson, in his definition of the term Coutchiching* described that formation as 

 lying non-conforinably below a basal conglomerate of the Keewatin, which conglomerate, he thought, represented 

 an important time interval and break in the Archean series. In Minnesota, however, no such conglomerate has 

 been found at the point of transition from the Keewatin to the mica schists supposed to be Coutchiching, 

 although careful examination and search were made with the special view of finding the break. Quite recently 

 Mr. Coleman has shown that the conglomerate to which Mr. Lawson referred is not at the bottom of the Keewatin, 

 but contains much Keewatin detritus and probably represents, as suggested by Mr. Coleman, a break in the 

 Keewatin itself. It is therefore comparable with the break which is well known in Minnesota, and on the 

 evidence of which the Keewatin is divided into upper and lower.f 



The occurrence of the first grand folding may or may not have brought the flexed 

 strata to verticality. The fact that both the earlier and the later beds are now in a 

 vertical position rather indicates that the first folding which took place with the first 

 granitic invasion was not sufficient to produce verticality. This uniformly vertical 

 attitude of the Archean is one of its most marked characteristics. It implies, of 

 course, that all the thicknesses are repeated, perhaps many times, in the measure- 

 ment of any extensive traverse, the original sharp upward flexures having been 

 denuded. In the case of the Upper Keewatin these flexures have not so universally 

 been productive of metamorphism, and indeed have not usually been attended by 

 granitic intrusion. The strata are compacted and in some measure a disintegrating 

 kind of metasomatic change has passed over them. So far as known, however, 

 they are not generally converted to gneiss and mica schist. The original minerals 

 and rock fragments are distinctly preserved. Where, however, these Upper Keewatin 

 strata are also penetrated by granitic dikes or by large bosses of granite, as in the region 

 of Snowbank and Disappointment lakes, they are rendered micaceous and can often 



* On the geology of the Rainy Lake region. Oeol. Sur. Canada, vol. iii, report F, pp. 5-183 (for 1887-88), 1889. 

 f Report of the Bureau of Mines (Ontario), vol. vii, p. 153, 1898, "On the clastic Huronian rocks of western Ontario." 

 American Geologist, xxi, pp. 222-229. " Some resemblances between the Archean of Minnesota and that of Finland." 



