454 THE GEOLOGY OP MINNESOTA. 



[Amygdaloid. Diabase. 



Ific. The section shows an ophitic rock, but interspersed in the mass of the 

 rock are areas filled with a very fine aggregately polarizing substance, and others 

 that are green in common light. The former are quite translucent in common light, 

 but nearly dark between crossed nicols, though crypto-crystalline. The latter, green 

 in common light, are occasionally so macro-crystalline that they lighten and 

 extinguish throughout their whole area. 



The green areas are largely derived from alteration of the original olivines, but 

 not wholly, for some of them appear to be associated with mtr/itc of the first genera- 

 tion, lioirlinjite is probably the mineral which, when greenish, or brownish, still 

 gives distinct general extinction. 



Thomsonite and some saponite (thalite) also are seen in this section. 



One section. 



Aye. Manitou (same as the rock forming Terrace point). N. H. w. 



No. 624. AMYGDALOID. 



Underlying No. 623. This bed, by its easy disintegration, forms two small harbors, one protecting from 

 the northeast, and one from the southwest. This bed is about eight feet thick, but varies some, as the characters 

 of No. 623 fluctuate up and down. 



Ref. Annual Report, x, page 59. 



Meg. Brownish red, fragile, charged with thalite and laumontite, both in the 

 rounded cavities and in veins. 



Mic. An ophitic structure is apparent, but some of the aiigites also preceded 

 fhe feldspars. The olivine is either entirely lost or remains only in the form of a 

 brownish mineral, resembling bowlingite. Occasionally a distinct cleavage perpen- 

 dicular to the extinction is visible in this mineral. 



One section. 



Aye. Manitou. ,N. H. w. 



No. 625. DIAHASK. 



Eclipse beach, sec. 26, T. 60-3 W. From this bed are derived the " thomsonites " of Eclipse beach. Thick- 

 ness cannot be made out, as it forms the coast line eastward. It has a close relationship with Nos. 623 and 624, 

 and in many places they probably could not be separated from each other stratigraphically. At this point, how- 

 ever, No. 625, in general, lies below Nos. 623 and 624. It is nearly, if not exactly, on the same horizon as the 

 amygdaloid at Terrace point, judging from the run of the beds as they extend along the coast from that place. 

 The same horizon extends to Poplar river without much deviation. 



Ref. Annual Report, ix, page 49; Annual Report, x, page 60. 



Meg. Therockitself is dark green, amygdaloidal, but notso easily disintegrating 

 as No. 623. It is blotched with fine and coarse thalite masses, which are frequently 

 coated with a red scale of some zeolite, perhaps of heulandite. It also embraces the 

 mineral which has been generally known as thomsonite, but which does not exhibit 

 the optic characters of that zeolite. This is sometimes in large masses. 



M!<: This rock, or others like it, has been described already. It is ophitic, the 

 olivine is not preserved, but has contributed to the formation of the abundant thalite. 



