PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 465 



Diabase.] 



has n e parallel with the fibres, and hence always positive elongation and parallel 

 extinction, with quadrangular basal sections in which n f runs to the angle, as seen 

 637 A /v*. * n fig ure 31, between the sides, as in mesotype. (See Dana's System 

 of Mineralogy, figure 1, page 600.) A micro-chemical test of this 

 gives only soda. Prof. Lacroix, in connection with this determin- 

 FIG. si. BASAL SEC- ation considers this specimen illustrates the same structure as 



TION OF MESO- 

 TYPE. figured by him in Mineralogie de France et de ses Colonies, figure 7, 



page 265. It is possible both slides are from this vein, but it is not probable. This 

 is the first identification of mesotype in Minnesota. 



Analysis by Prof. J. A. Dodge resulted as follows: 



Oxygen. Ratio. 



Silica, 47.25 25.19 6.47 



Alumina, 24.78 11.54) 



Oxide of iron, .48 .14J 11>bb 



Lame, 1.23 .35] 



Magnesia, .71 .28 j- 4.52 1.16 



Soda, 15.05 3.19 j 



Potash, traces 



Water, 10.37 - 9.22 2.37 



99.87 



N. H. W. 



No. 638. DIABASE. 



Rock at the base of Encampment island, on the north side. Compare No. 106. 



Ref. Wadsworth, Bulletin ii, page 110; Annual Report, ix, page 28; Annual Report, x, page 64. 



Meg. A dark, heavy diabase, rather coarse. 



Mic. The ophitic structure is conspicuous, and green spots appear to have 

 resulted from alteration of non-differentiated portions of the magma, as remarked 

 by Wadsworth. The spherulitic polarization which appears in some places in this 

 slide is probably produced by a zeolitic mineral, such as scolescite. This radiated 

 fibrous zeolite is surrounded by another, which is probably thoiusonite, as its confused 

 fibres are elongated, sometimes positive and sometimes negative, and its double 

 refraction is near that of thomsonite. 



One section. 



Aye. Cabotian. N. H. w. 



No. 639. DIABASE (imtholivine). 



One mile east of the mouth of Silver creek. Forms a high bluff (see Nos. 105, 819.) 



Ref. Wadsworth, Bulletin ii, page 104; Annual Report, ix, page 28; Annual Report, x, page 64. 



Meg. Similar to the last. 



Mic. Augite is ophitic with respect to the feldspars, and is fresh; also, in the 

 same manner it surrounds the olivines, which are considerably altered to a yellowish 

 green, sub-opaque, or sometimes brownish substance, which frequently goes by the 

 name of serpentine. There is also a considerable amount of the same greenish alter- 

 ation product which is referable to the original magma. This is ophitic in its relation 



81 



