244 THE GEOLOGY OP MINNESOTA. 



[Diabase. Shale. 



No. 180. DIABASE. 



From the middle island at the mouth of Poplar river. 

 Ref. Annual Report, ix, page 48. 



Met/. A yellowish-brown, fine-grained, diabasic rock, permeated with a soft, 

 yellow mineral (saponite). The abundance of this mineral gives the yellowish color 

 to the hand sample. There are two small amygdules of calcite, and one corner of 

 the specimen has a mass of calcite, heulandite and a black mineral. 



Mir. The section shows a fine-grained diabase with abundant feldspar, which, 

 by its extinction angles, is seen to be l/ibr/i<//-!tc, a comparatively small amount of 

 HiKjUe in small plates later than the feldspar, much hematite, some magnetite, and a 

 very little thalite. The section contains very much less thalite than would be indi- 

 cated by the hand specimen, and, contrary to expectation, the section shows a com- 

 paratively fresh rock. 



One' section. 



.\ijr. Manitou. u. s. e. 



No. 181. DIABASE. 



"Underlies No. 180 and does not vary much from it, except in being more evenly and more thinly bedded; 

 and in separating into closer joints, so as to disintegrate, leaving No. 180 to stand alone, and really causing its 

 more rapid demolition. Nos. 180 and 181 form substantially one rock, and are both what has been styled trap 

 along here. In weathering they become very rusty, when not under friction, and brick red, crumbling in little 

 red globules. These beds are twenty-four feet thick.'' 



Ref. Annual Report, ix, page 48. 



Meg. The rock is a fine-grained, dark brown, diabase, with considerable dissem- 

 inated yellow saponite (thalite} , similar to that in No. 180, but not as abundant. There 

 are a few small calcite amygdules, and some soft red areas, which perhaps represent 

 altered olivines. Another specimen with the same number is of a similar rock 

 stained red. 



Mic. An ordinary diabase with the plagioclase and auyite comparatively fresh, 

 magnetite and abundant hematite in grains. The section is full of cracks which have 

 been filled with hematite, and it is to this abundant hematite, both in grains and in 

 the cracks, that the color of the red hand sample is due. 



One section. 



Age. Manitou. u. s. G. 



No. 182. SHALE. (Red.) 



" Is directly under No. 181, and is a shaly, red, easily crumbling rock, apparently of not uniform thickness, 

 but in one place is about eight feet thick; on the east of Poplar river associated with a red conglomerate." 

 Ref. Annual Report, ix, page 48. 



Meg. A soft, red, sandy shale, with gray blotches (apparently of laumontite) in 

 the cracks. 



No section. 



Age. Potsdam. u. s. G. 



