362 C. W. HALL KEEWATIN OF EASTERN AND CENTRAL MINNESOTA 



several places around Thomson and Cloquet, represent the existing 

 alteration stage. 



THE CLA Y SLA TES 



These are extremely finely crystalline rocks occurring in bands from 

 2 or 3 to many feet in thickness between the layers of graywacke and 

 graywacke slate. The difference between the clay slates and those ap- 

 parently lies in the effect which pressure and shearing have left upon 

 them. The slates have a well defined cleavage, so com})lete that con- 

 siderable quarr3'ing has been done, while the graywackes have a typical 

 schistcfsity, the more perfect as the rocks are more thoroughl}^ altered. 

 The two are seen side by side in many situations, as plate 31, figures 

 1 and 2, clearly shows. 



The texture of the clay slates is so extremely fine that the mineral 

 composition cannot satisfactorily be determined. In comparison with 

 the commercial product of the Slatington quarries of Pennsylvania, no 

 marked difference was seen. Series of slides made from the ordinary 

 roofing slates of Thomson and Cloquet and from the graywacke series 

 gathered at different points w.ithin this district gave texture the chief 

 distinction to be made. The fundamental difference is no doubt one of 

 chemical composition, brought about by the variation in the size of 

 grain and consequent transportation at the time sediments were deposit- 

 ing; The transverse cleavage characterizing the slates is due to move- 

 ments produced by lateral compression. This force h«is been sufficient 

 to produce the cleavage phenomenon in the fine sediments through their 

 capacity for microsco[)ic faulting, and to place in vertical direction the 

 carbonate concretions which occur both in slates and associated gray- 

 wackes (see plate 31, figure 2). 



THE HORNBLENDE GRAYWACKES 



These rocks differ from the foregoing graywackes in the extent to 

 which alteration has progressed. Quartz which appeared in those as 

 well preserved rounded grains here has to a large degree passed into a 

 microcrystalline stage. Feldspars appear in rare and isolated fragments 

 and in areas of finely crystalline alteration products, namely, quartz, 

 calcite, siderite, muscovite, and hornblende. The figure selected for 

 illustrating this phase of the alteration is one in which hornblende is 

 developing a large area in crystallographic continuity out of what are 

 regarded as grains of feldspar and quartz. Numerous globular dark 

 grains, identified as magnetite, are scattered through the secondary por- 

 tions of the field. More rarely than in the preceding are seen fragments 

 of the earlier rocks out of whose degradation these hornblende gray- 



