398 THE GEOLOGY OP MINNESOTA. 



[Diabase. Sandstone. 



Meg. A very fine-grained, reddish-brown rock, holds a very few small porphy- 

 ritic feldspars. Some small dark dots, probably magnetite, are visible. 



Mic. The sections are much charged with magnetite and are highly stained by 

 hematite, thus considerably obscuring the other minerals, which are quartz and 

 //hlx/Hir. The feldspar is in small lath-shaped crystals and in larger grains, but the 

 main mass of the rock, aside from the iron ores, seems to be composed of secondary 

 quartz, often in small poikilitic areas. The exact nature of the rock is not clear, 

 but as it seems to be similar to No. 522, it is thought to be a rather basic rock, largely 

 glassy originally, but now much charged with secondary quartz. It is not, however, 

 improbable that the rock was originally more of a trachyte than a basalt. 



Two poor sections. 



Age. Cabotian. u. s. G. 



No. 524. DIABASE (urith olivine). 



Splitrock point. The great sheet of dark, basic rock that embraces the anorthosyte pieces. This piece 

 was obtained near the large anorthosyte block that forms Splitrock point. This rock can be seen, at places along 

 here, to be underlain by Nos. 522, 523, and the Two Harbor rock (No. 117) in the order named. At Splitrock 

 point this sheet is seventy-five feet thick (see No. 112). 



Ref. Annual Report, ix, pages 29, 30; Annual Report, x, page 40. 



Meg. A massive, homogeneous, dark-green rock. 



Mic. The (titgifc has the ophitic relation to the/f/f/.sy^rx. The fil'icine* are small. 

 One section. 



Age. Cabotian. N. H. w. 



No. 525. SANDSTONE. 



Associated with some of the amygdaloidal parts of Nos. 522 and 523. 

 Ref. Annual Report, x, pages 40, 114. 



Meg. Apparently a grit composed of grains of de vitrified glass similar to 

 rocks Nos. 8A or 17, but mingled with masses of considerable size of amygdaloid. In 

 the sand rock which also varies to a shale, and is light-colored as well as red, are 

 flat, water-laid surfaces with an undulatory parting which has impressions resembling 

 fucoids. 



No section. 



Age. Cabotian (?) 



Remark. This rock is less firm and less siliceous than most of the grit rocks 

 derived from apobsidian (as Nos. 17 and 30). Its shaly structure also is different from 

 anything seen as yet in the Cabotian, and it might therefore be placed higher in the 

 series. It is referred to the Cabotian rather than to the Manitou because of its asso- 

 ciation with the eruptives Nos. 522 and 523, which are immediately connected 

 stratigraphically with No. 117 (the Two Harbor rock), and all of these have been 

 referred to the Cabotian. Owing to the complexity of the structure, and the 



