Sources of the Constituents of Minnesota Soils. 401 



The calcareous and siliceous shales. — The fourth group is 

 that of the calcareous and siHceous shales, partly Cambrian, part- 

 ly Cretaceous and partly Glacial in age. While they are at the 

 present time of no great extent, before the Glacial period they 

 were probably spread over many square miles of Minnesota. 

 They must have formed the underlying rocks over a considerable 

 belt in southern Minnesota where the edges of the Cambrian rocks 

 came to the surface, and to the west and northwest, where the 

 Cretaceous — Fort Pierre shales of the Dakotas — extended into 

 the state, a great extent of our territory must have been covered, 

 possibly more than half the state. The soft and friable condition 

 of these rocks caused them to be easily eroded by the ice which 

 was pushed down from the north during the Glacial epoch. 



The calcareous shales possess some peculiar, and for the ag- 

 riculturist, valuable properties. Chemically, as will be seen, they 

 contain in large measure some of the essential elements of plant 

 food, and they are partially soluble; physically, they are not so 

 heavy and compact as clays, they are easily broken up, allow 

 roots to push into them, and they crumble and become finely pul- 

 verized under the action of sun and rain. The soil produced by 

 them is not a heavy clay, but rather a mingled clay and sand, — 

 and they will doubtless make, under some circumstances, an ex- 

 cellent loamy soil. 



The siliceous shales contain a far less amount of soluble ma- 

 terial than do the calcareous shales. There ajre two types of these 

 shales in the state: those that stand intermediate between the 

 dolomites and sandstones of the southeastern Minnesota suc- 

 cession and those which are directly or indirectly due to the ice of 

 the Glacial period. The first type represents a rock originally 

 very different from the existing one, the change being effected 

 by combined solution, erosion and transportation. The existing 

 condition may be regarded as a transition from a rock of quite 

 diverse character towards a very clean bed of sandstone. 



In the following table of analyses are included the decom- 

 posed granitic rocks which occur in several portions of the state. 

 Along Birch Cooley, in the Minnesota river bottoms below Red- 

 wood Falls, at Granite Falls, and elsewhere, these rocks are an 

 important soil constituent. 



XXIII. Decomposed gtieiss, Birch Cooley; analysis by A. 

 D. Meeds. 



