274 THE GEOLOGY OF MINNESOTA. 



[Basalt. 



evidently originally glassy, or nearly so, and it has been considerably altered from 

 its original condition. 



One thick section. 



Age. Manitou(?) u. s. o. 



No. 237. BASALT (with olivine). 



From a curious isolated mound of thin-layered, almost slaty, rock, with curving lamination, which rises 

 eighteen feet on the beach, or between the beach and the lake, a short distance east of No. 236. Shaped like a 

 haystack. (See Nos. 1824-1826.) In structure and lamination reminds of a sedimentary rock, but its amygda- 

 loids show it is an eruptive. 



Ref. Annual Report, ix, page 59. 



Mi'ij. Compact, fine-grained, brownish diabasic rock. Evidently distorted and 

 curved by pressure. 



Mic. The little feldspars are mostly idiomorphic, lying in a nearly isotropic 

 brown glass which still renders a confused polarization. 



Magnetite, in very fine particles, is common. 



Olivine appears largely replaced by magnetite, yet, occasionally in the centre of 

 a magnetited grain, a green decomposition product remains. The olivines, thus 

 changed, still show their original idiomorphic outlines. They appear occasionally 

 to break the symmetry of the feldspars, but in general the still mobile magma was 

 so abundant that the feldspars found room to form entire without being trenched 

 upon by the earlier olivines. 



Hematite, and apparently chlorite, resulted from alteration. 



One section. 



Age. Cabotian. N. H. w. 



No. 238. BASALT (with olivine ). 



Same place as No. 237, but from a slaty condition of thfs rock. 

 Ref. Annual Report, ix, page 59; Annual Report, x, page 47. 



Same rock as No. 237, except that it contains a little quartz, and occasionally 

 a porphyritic feldspar of the first consolidation; entirely reddened by oxide of iron. 

 One section. 



Age. Cabotian. N. H. w. 



No. 239. BASALT (with olivine). 



The same rock as No. 238, but non-fissile, firm, weathering brown, rather finer 

 grained. 



One section. 



Age. Cabotian. N. H. w. 



No. 240. BASALT (with olivine ). 



Prom the same place as No. 238, and from the same rock. 

 Ref. Annual Report, ix, pages 59, 60. 



. Finer grained, breaking conchoidally. 



