398 Sources of the Constituents of Minnesota Soils. 



when the rock is mostly silica, to a clayey soil on the other, when 

 alumina occupies the largest place in the chemical composition 

 of the rocks. 



Basic eruptive rocks. — The second group, or the basic erup- 

 tive rocks are important, first, because of their extent, for prob- 

 ably ten thousand square miles of the commonwealth is underlain 

 by these ; and secondly, because of their chemical qualities. They 

 possess the same chemical constituents as the first class considered, 

 but in quite different and more varying proportions. They con- 

 tain less silica and more of the alkalies and alkaline earths, the 

 real plant food than do those. For instance, the lime in a cubic 

 mile of these rocks, if none of it were wasted or used in other 

 ways, is sufficient to grow a crop of grass yielding two tons per 

 acre for hundreds of years. When these rocks are weathered and 

 the debris is gathered by erosion and transported into the mead- 

 ows and valleys of the northern portion of the state, a soil of great 

 richness is produced ; grasses, sedges, shrubs and forest trees grow 

 A^gorously. 



In area these eruptive rocks underlie about one-fourth of 

 Minnesota to the north and west of the line just drawn from Tay- 

 lor's Falls through Anoka, New Ulm and Tracy to South Dakota. 

 The great field of volcanic activity was almost the entire tract 

 north of Lake Superior in Minnesota. Thousands of square 

 miles were overflowed by lava beds and the exudations of hun- 

 dreds of dikes which welled up from the deep-seated plastic rocks. 

 Cracks in the old rocks some of them of huge extent were made 

 across the state to the south and southwest and filled with this 

 plutonic matter. They can be seen today in almost every county 

 where the granitic and gneissic rocks appear. The chemical com- 

 position of the basic eruptive rocks will appear from the following 

 selected analyses, also made in the laboratories of the State Uni- 

 versity : 



VII. Medium grained diabase, north shore Lake Superior; 

 mean of five analyses ; analysis by H. B. Greeley. 



VIII. Thomsonite-bearing diabase, north shore Lake Su- 

 perior ; analysis by Prof. C. F. Sidener. 



IX. Columnar diabase, Grand Marais, north shore Lake Su- 

 perior; analysis by Professor C. F. Sidener. 



X. Porphyritic quartz diabase. Saint Augusta, Stearns 

 county ; analysis by A. A. Finch. 



