132 THE GEOLOGY OF MINNESOTA. 



[Diabase. Calcite. 



fibrous chlorite. The cavities of the rock are in part filled with epidote and a little 

 chlorite, and the former mineral is found in abundance in the groundmass. The 

 groundmass shows no evidence of having contained augite, and it may have been 

 originally glass. No. 36, however, of which 36A is a part, contains augite. 

 One section. 



Aye. Cabotian. u. s. G. 



No. 37. DIABASE (?) 



Duluth. At the shore immediately eastof Chester creek, standing up like a dike. 

 Kef. Annual Report, ix, page 16. 



Meg. Compact, fine, basaltiform, with some large geodic concretions; pyritif- 

 erous; extends four rods. This rock is not essentially different from No. 36, but has 

 spots of calcite and epidote. It is more decayed and contains more numerous gray 

 grains that are semi-opaque and seem to be lencoxene. 



One section. 



Aye. Cabotian. N. H. w. 



No. 37A. CALCITE. 



Duluth. From a concretion in No. 37. 

 Ref. Annual Report, ix, page 16. 



Meg. The hand specimen is a large mass of well crystallized calcite. On one 

 side of the specimen is some epidote and hematite. 

 No section. 

 Age. Concretion in Cabotian eruptives. u. s. G. 



No. 38. DIABASE. 



Prom a dike east of the mouth of Chester creek. This dike runs north and south, and hangs toward the 

 east 10. It is eighteen feet wide. On the west side the adjoining rock has been hardened and rendered similar 

 to the dike in external aspect. This dike cuts the foregoing eruptive sheets. 



Ref. Annual Report, ix, page 16. 



Mac. The rock is fresh and gray, medium. grained, evidently a basic rock of 

 the gabbro family. 



Mic. The feldspars are embraced ophitically by the pyroxenes. A section per- 

 pendicular to the bisectrix ( g ) gives extinction on cleavage at 25 to 28, and on macles 

 at to 2. The optic plane, determined by the greatest curvature of the hyperbola 

 in a section nearly perpendicular to an optic axis, makes an angle, with cleavage, 

 of about 12. If this section be near 001 the measure indicates Idbrador-bytownite. 

 If it be near 100 the indication lies between <nitlexine and /i/bnu/orife. A tabular 

 section showing distinct cleavage is taken for 010, and extinction on it is 25 and on 

 another 30, also pointing to labrador-bytownite. (This cleavage, in the first case, is 

 not parallel with the macle. Many of the microlitic feldspars are crossed by such 

 cracks, sometimes at right angles to their elongation and sometimes obliquely.) 



