402 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



No. 529. D DIABASE (or gaWro). 



Beaver bay. From the peninsula which forms the southern barrier to enclose Beaver Bay harbor. 

 Eef. Annual Report, x, page 41; Bulletin ii, page 117. 



Meg. Dark, diabasic rock, apparently from the great sheet which carries the 

 feldspar masses. 



Mic. The auf/lte -was in part earlier than the feldspar, and is considerably 

 altered, sometimes to a grayish mineral that resembles leucoxene, but more frequently 

 to a greenish chloritic substance. When it is cut favorably and is sufficiently fresh 

 it shows a diallagic lamination parallel with 100. There is considerable quartz in 

 pegmatitic growth in the feldspars. 



Two sections. 



Age. Cabotian. 



.Remark. The samples collected represent a series of six numbers, designed 

 to illustrate the progressive stages in the decay of this rock to a surface soil. It is 

 evident that the freshest of the samples collected is yet considerably altered, though 

 probably not by weathering, the chief change consisting in the entrance of quartz. 

 This is more likely to have entered the mass while it was still hot. N. H w. 



No. 530. DIABASE. 



From the shallow bay half a mile west of the west point of Beaver bay. This rock contains patches of red 

 rock like No. 526, and within two rods, after a short interval not exposed, the rock No. 526 is seen in full force, 

 forming a bush-covered bank. The patches are as fragments or boulders, generally, but some patches do not 

 appear like transported masses, but like dikes and veins. Forms a narrow dike-like belt two and a half feet 

 wide, running east and west. 



Ref. Annual Report, x, page 41. 



Meg. Dark, rather fine-grained diabase. 



Mic. The section shows an ordinary ophitic diabase, with some of the augifc 

 earlier than the plagioclase, and with a little quartz and biotite. The augite is the 

 gathering place of much magnetite in fine grains, and the gray leucoxene mentioned 

 in Nos. 529 and 531 as a result of alteration indicates that it is titaniferous. 



One section. 



Age. Cabotian. 



Remark. The irregular inclusions of the "red rock" in this diabase, although 

 they appear, in some instances, like veins and dikes, are not to be taken as evidence 

 of the later date of the red rock. They illustrate the easy dissemination of the acid 

 elements of the red rock in the basic, a fact which is exemplified in many places, and 

 which has been noted by Bayley on Pigeon point.* On the other hand the isolated, 

 scattered pieces of the red rock in the diabase, which in some places are nearly as 

 abundant, but never so large, as the pieces of anorthosyte, can be explained only by 

 assuming the greater age of the red rock. N. H. w. 



Bulletin cix, U. S. Oral, tiurvry. 



