THE GEOLOGY OF MINNESOTA. 



[Diabase. 



NO. 160. DlABASlO. 



"From the point that protects Little Marais from the east, and occupying, in the form of basaltic trap, the 

 coast for two and a half miles further east, rising in some places about 100 feet, the conglomerate sometimes 

 rising fifty feet above the lake, making a bold and dangerous strip of coast for small boats. The two interlock 

 and blend in stratification, and the conglomeratic characters particularly become confused, and even lost, appar- 

 ently passing into amygdaloid. The dip is toward the lake in the main, but there are spots where the dip is 

 invisible. Those extend to and beyond the Manitou river (see Xos. G28 and 029)." This has a conspicuous 

 basaltic structure. 



Ref. Annual Report, ix, page 41; Annual Report, x, pages 42, 61, 03, 139; Proceedings American Associ- 

 ation for the Advancement of Science, vol. xxx, page 162. 



Meg. A brownish, diabasic rock of rather fine grain, more or less permeated by 

 reddish substance, appearing like a stain. 



Mir. A diabase, composed otplagioclase, nnf/ilc, mtii/nr/ifr, /triiinfi/f, c/tloritv and 

 a reddish alteration product which may represent iii/i>i</ir!ilnri/i,:rf/ DUHJHIU. There, 

 are some larger areas of feldspar which appear somewhat like porphyritic crystals, 

 but without good crystal boundaries. Under polarized light these areas break up 

 into aggregations of the usual feldspar laths. 



One section. 



Age. Manitou. u. s. G. 



No. 161. DIABASE. 



From the shore at the town line between ranges 5 and 6 (on section 36), east of Pork bay. Apparently a 

 layer in the midst of brecciated amygdaloid and conglomerate. Some of this breccia holds what appears like 

 mesolite and thalite; other parts hold calcite and perhaps prehnite. 



Ref. Annual Report, ix, page 42; Annual Report, x, page 140. 



Mi'//. These parts are not evenly disseminated, but often are found in patches 

 or lumps closely aggregated, the rest of the rock having less. 



Mir. In general this is an ophitic diabase, much decayed, consisting of a 

 plagioclase (probably labniiloritr) <li<i//(//', nutyi/rfife and olivine, with their products 

 of decay. The change in the olivine has produced, generally, a hematitic red, 

 amorphous, opaque substance. Disseminated abundantly throughout the rock, 

 though not so abundant in the thin section examined, is the soft, greenish or grayish 

 soapy mineral which was named tJui/ifv by Owen, but which is regarded by Dana as 

 a form of mi/>i>//ifr. It is a secondary product, and has gathered in the cracks of the 

 rock. It has not the farm of amygdaloidal filling, but is found finely distributed 

 among the other minerals. From what mineral, or minerals, it is chiefly derived, 

 is not evident, but it is highly probable that the olivine was largely instrumental 

 in its formation. It is more fully discussed in connection with other numbers where 

 it is more favorably exposed and occurs in larger quantity. (Compare Nos. 91B, 162 

 and 193.) 



One thick section. 



Aye. Manitou. N. H. w. 



