332 C.W.HALL — KEWEENAWAN AREA OF EASTERN MINNESOTA 



related to the arrangement of minerals with each other somewhat as 

 gneisses differ from granites. 



The amj^gdaloids are not peculiar. They var}^ greatly, of course, in 

 frequency of vesicles and in the mineral species filling the same. 



Textured characters. — The principal factor in determining the texture of 

 the diabases would seem to be the thickness of the several flows of the 

 diabase succession. In the field it is seen that the flows measuring but 

 a few feet in thickness are finely textured. There is nowhere in the 

 midst of the flows any trace of an existing or of a past vitreous condi- 

 tion beyond the finel}'' crystalline and greatly altered bottom layers, 

 which bear frequent evidence of devitrification. The porphyritic habit 

 is often developed. This seems to be more characteristic of the thicker 

 flows and the amygdaloidal layers than of the more usual rock masses 

 of compact habit. 



Lithologic characters. — Lithologicall}^ considered, the rocks must be dis- 

 cussed under two divisions, clastic and eruptive. Incidentally the entire 

 area is brought in review. 



The elastics are, first of all, conglomerates. While exhibiting a wide 

 range in texture, they occur in several well defined beds, which are in 

 the profiles carefully indicated. Their color is prevailingly red or reddish 

 brown. They represent a wide range of material — diabases, amygdaloids, 

 diabase ])orphyrites, gabbros, granites, felsites, quartz porphyries, quartz- 

 ites, sandstones, and others. Boulders as large as 20 and 24 inches in 

 diameter occur, but usually of average size. It is seen at once that this 

 material was derived from a comparatively large geographic area, as well 

 as a wide petrographic province. All the rocks named occur in the Lake 

 Superior basin and upper Mississip})i valley, but several of the species 

 are not now known to occur within 50 and 75 miles of these boulders, 

 nor have they been seen an3^where to the south of the beds. Other rock 

 species — that is, vitreous quartzites — again, have been seen nowhere to 

 the north, but they do occur in extensive formations east and southwest 

 of this locality. . No fossils have thus far been found. Native copper is 

 present, but in such small (juantities that mining operations have never 

 passed beyond the exploratory stage. 



Doctor Berkey was the first to note the occurrence of clastic rocks 

 within the eruptive flows at Taylors Falls and Saint Croix Falls. Instead 

 of intraformational elastics, he considered them simply as breccias, 

 formed intermittentl 3^ throughout the period of volcanic activity.* The 

 breccia as a rock t3^pe is very obscure, since it is as hard and compact 

 as other portions. Close inspection discloses that the fragments of dia- 

 base are irregular, of all sizes, and lie imbedded in a matrix of finely 



♦ Geology of the Saint Croix Dalles. Amer. Geologist, vol. xxi, 1898, pp. 145, 146. 



