September 2, 1910] 



SCIENCE 



313 



Of the several dozen analyses made in this 

 study, it has been possible to select those of 

 Table I as representing extensive types and it 

 is desirable to have them on record. Atten- 

 tion may be called to certain peculiarities in 

 these rocks as compared with world averages. 

 The calcium is uniformly high compared with 

 magnesia. In the average Minnesota rock 



soda is predominant over potash, but there are 

 some decided exceptions to this. A similar 

 discussion of Wisconsin igneous rocks, south 

 of the Keweenawan area, by Weidman' shows 

 essentially the same peculiarities, even more 

 extreme. Present data are rather insufficient 

 to yield a serviceable estimate of the average 

 igneous rock for the whole state. In the ehem- 



TABLE I 



Rock Analyses and Averages. {By F. F. Grout.) 



1. Typical red granite of central Minnesota. 

 T. 124 N., R. 28 W. Contains quartz, soda-ortho- 

 elase and microcline, a little oligoclase, biotite 

 and hornblende, and accessoiy magnetite, apatite 

 and sphene. Augite has been found as cores in 

 the hornblende. 



2. Typical gray granodiorite of central Minne- 

 sota. T. 124 N., R. 28 W. Contains quartz, 

 oligoclase. orthoclase, microcline, augite and horn- 

 blende with accessory ilmenite, magnetite, apa- 

 tite, sphene and zircon. Hornblende and in some 

 cases biotite develop from augite. Much of the 

 quartz and feldspar is secondary and enlarged 

 original crystals. 



3. Approximate average composition of Minne- 

 sota granites from all available analyses. (17 

 new analyses included.) 



' Wisconsin Survev, Bulletin X\l. 



4. Mottled diabase, Keweenawan of Taylors 

 Falls. T. 34 N., R. 19 W. Contains altered 

 augite and plagioclase with ophitic texture; 

 olivine and magnetite. The alteration gives much 

 chlorite and epidote. 



5. Hackly diabase, Keweenawan of Snake Ri\-er. 

 T. 39 N., R. 21 W. Similar to the preceding num- 

 ber, with less augite, thus leaving the texture 

 diabasic. 



6. Conchoidally fracturing diabase, Keweena- 

 wan of Crooked Creek. T. 42 N., R. 18 W. Sim- 

 ilar to No. 5, with so little augite that the texture 

 is granular. 



7. Estimate of the average composition of Ke- 

 weenawan lavas from all analyses available from 

 the Lake Superior Region. (15 new Minnesota 

 analyses.) 



