140 THE GEOLOGY OF MINNESOTA. 



[Crystalline rocks. 



to 71.81 per cent. Another analysis of a sample from Duluth gave the fol- 

 lowing results.* 



Silica, 75.78 



Alumina, 11-09 



Sesquioxide of iron, 2.09 



Calcium oxide, .86 



Magnesium oxide, .65 



Potassium oxide, - 1.06 



Sodium oxide, - 6.43 



Water, 1.82 



99.78 



At the two points mentioned this red syenite is fine-grained; but from the 

 occurrence of occasional boulders of very coarse grain, evidently from the 

 same formation in its northeastern extension, along the shore of lake Supe- 

 rior, it is believed that this rock affords a very beautiful and coarsely crys- 

 talline building stone.f Besides quartz this rock contains red orthoclase, 

 hornblende (often changed to chlorite), magnetite, apatite and ferrite. 



In the Minnesota valley, extending from near New Ulm to Big Stone 

 lake, are numerous exposures of crystalline rocks. Sometimes these rocks 

 present favorable opportunities for the prosecution of this industry, but 

 they differ considerably from those of Stearns, Benton and Sherburne coun- 

 ties. They are generally gneissic, instead of massive. They are more fre- 

 quently true granite. They are always red. While their laminated structure 

 renders them more easily wrought, and thus gives them an advantage over 

 the firm gray syenites, of the Mississippi valley, it also renders them softer 

 and more destructible under the action of the weather. They seem to have 

 less quartz, and more of the cleavable minerals orthoclase and mica. Still 

 there are exceptions to the gneissoid structure of the Minnesota valley 

 granites, as may be seen in some of the exposures at Big Stone lake, and in 

 the railroad-cuts near Montevideo. 



At East St. Cloud and Watab there is still another variety of syenite 

 (No. 9), which, however, is probably only a coarser crystalline condition of 

 the fine-grained gray syenite (Nos. 1 and 4),J since on analysis it has about 

 the same content of silica, alumina and iron. It contains more lime and 

 magnesia, but less potash and soda. It consists essentially of the minerals 

 quartz, orthoclase, plagioclase, hornblende and biotite. At East St. Cloud 



'Tenth annual report, p. 204. 



tC'ompare Nos. 667, 668 and 673 ; also 685 and 68, tenth report. 



{Nos. 801 and 805, p. 106, Tenth annual report. 



