778 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



No. 1760. DIABASE. (Uralitic.) 



From the top of West Twin peak. The country rock. 

 Ref. Annual Report, xxi, page 158. 



Meg. Fine grained, without boulder forms. 



Mic. This rock consists, like No. 1758, of uralite and plagioclase, but was originally 

 not ophitic. On the other hand the augites were earlier than the feldspars and were 

 small. One section. 



Aye. Archean (Keewatin). N. H. \v. 



No. 1761. DIABASE. (Uralitic.) 



Same locality as No. 1760. 



Ref. Annual Report, xxi, page 158. 



Meg. A medium-grained, uralitized diabase. No section. 



Aye. Archean (Keewatin). u. s. G. 



No. 1762. DIABASE (with olivine). 



West Twin peak, central part of a dike ten feet wide, cutting No. 1760, running northwest. 

 Ref. Annual Report, xxi, page 158. 



Meg. Evidently diabasic. 



Mic. With the ophitic structure are preserved also remnants of the augite and 

 of the olivine. Some of the augite was as early as the feldspar and some of it is 

 largely uralitized. The olivine shows the alteration to bowlingite. One section. 



Age. Dike in the Keewatin. N. H. w. 



No. 1763. DIABASE (withoHt-inc). 



West Twin peak, near the edge of the same dike as No. 1762. 

 Ref. Annual Report, xxi, page 158. 



Meg. Dense and dark colored. 



Mic. Like the last, but finer grained. The rock can hardly be called ophitic, 

 since the augites are small and as early as the feldspars. This rock has more matjuc- 

 tife, less olivine, and is preserved better, showing, however, a little biotite. One section. 



Age. Dike in the Keewatin (possibly Cabotian). 



Remark. Being a part of the same dike as No. 1762, this slide shows an inter- 

 esting contrast, in having its augites earlier than the feldspars, or cotemporary with 

 them, never revealing an ophitic structure. It is apparent hence that the ophitic 

 structure is not necessarily an intrusive character, nor the absence of it a character 

 of massive rock. From the slides examined, not including Nos. 1762 and 1763, it is 

 apparent that the central mass and summit of the West Twin peak consists of igneous 

 rock. It is believed to represent the oldest rock known in the state, and to have 

 been a part of the original crust of the earth, the Kawishiwin of the Keewatin. 



N. H. w. 



