LITHOLOGY OF TttE SERIES 359 



way the rocks at Little Falls have been regarded as stratigraphically re- 

 lated to the " Thomson slates/' No evidence has been adduced, either 

 in their structural or mineralogic relations, to confirm this view. Again, 

 the granites at Saint Cloud, Sauk Rapids, and Watab have been held to 

 be Archean, and it was assumed further that could a contact be found 

 beneath the drift, it would disclose the Little Falls " slates " as lying 

 unconformably on the granites. The granites of the Saint Cloud area 

 were held to be of the same age as the gneisses and gabbro schists of the 

 Minnesota River valley. It was also thought that the Keweenawan erup- 

 tives at Chengwatana, Taylors Falls, and other places were poured out 

 and spread over the slates and schists as lavas cover older rocks in 

 every other region, and that the Cambrian sandstones stretched through 

 the upper Mississippi River valley northward until the worn edges of all 

 the underlying pre-Cambrian formations were covered by them over 

 thousands of square miles, into the very heart of the Lake Superior 

 synclinal trough. 



THE GRAYWA CKES 



These rocks occur around Thomson, Carlton, and Cloquet in extensive 

 exposures, typical in the village of Thomson. Their prevailing color is 

 a dark gray, which on weathering fades almost to white. The variation 

 in mineral composition is clearly evidenced on the weathered surfaces. 

 The rocks are everywhere shattered and fractured through crustal move- 

 ments until it is practically impossible to quarry blocks of satisfactory 

 size. Even the slates are so fractured and warped that only three or four 

 localities in the entire district have been found where plates of sufficient 

 size for commercial purposes can be extracted. Plate 32, figure 1, shows 

 this shattered condition near the railroad depot at Thomson, where giacia- 

 tion has smoothed the hardened gray wacke surface. Here the quartz, re- 

 sisting corrosion, stands out in rounded or etched grains, while numberless 

 pits which the surface carries represent the former resting places of the 

 more easily decomposed carbonate and silicate constituents. The grains 

 vary greatly in size ; the largest are the size of marrowfat peas, the others 

 diminishing until a texture of slaty fineness is attained. As a rule, the 

 finer the texture the darker the color, the slates being very black. The 

 rock is thoroughly indurated. It has a harsh feel when broken across 

 the planes of foliation. The individual grains are so cemented that a 

 conchoidal fracture and non-granular habit characterize the more massive 

 beds. The thickness of these beds could not be measured, outcrops 

 showing from 50 to 100 feet each ; hence the total must reach thousands 

 of feet. 



In mineral content, the coarser varieties, or the graywacke proper, 



LI— Rui.T,. Geot,. Soc. Am., Vol. 12, 1900 



