PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 189 



Diabase.] v 



in preexisting cavities. It is, at least, about on the horizon of the rock at Knife 

 river. 



Age. Cabotian. N. H. w. 



No. 126. DIABASE. (Coarse.) 



From about one-half mile up the creek, which enters the lake at Black beach, a few miles west of Beaver 

 bay, about the centre of sec 22, T. 55-8. This rock by its own disintegration furnishes the titanic iron sand of 

 the beach. 



Compare rock Nos. 10G and 107, etc., to which this rock may be referred, structurally and petrographically. 



Ref. Annual Report, is, page 32; Annual Report, x, page 139; Annual Report, xv, page 213; Bulletin ii, 

 pages 76, 77; Bulletin vi, pages 140, 420; American Association for the Advancement of Science, vol. xxx, page 162. 



Mi'//. Coarsely crystalline diabase, of a gray color, glittering with metallic 

 reflections of magnetite and the schillerizations of the pyroxene. 



Mic. Beautifully ophitic rock, the coarse plagioclases, somewhat decayed 

 showing albite, pericline and Carlsbad twinning. A grain cut perpendicular to a 

 bisectrix proves to be n e (c) and its extinction angle is 28, which falls between 

 lbi-(i<lt-ite and labradorite-bytownite. It is hardly necessary to search for further 

 determinative characters. This seems to be the character of the plagioclase in these 

 basic eruptives, almost without exception, at least in normal circumstances. 



AiKjitc. This mineral seems to take on the schillerization when it begins to 

 decay. That this is augite and not hypersthene, is evident from its strong refraction 

 and the frequent appearance of an optic axis in the zone 001:100. In one section a 

 bisectrix (n p ) is in the field with an optic axis. By means of the apparatus of 

 Lacroix* this distance is measured, and the optic angle in air is found to be approxi- 

 mately 85 8 ', which would give 2 V=48 , if the index of refraction on n v be taken 

 at 1.712.f Not much value attaches to this result, owing to the necessity of operating 

 on the half angle and the obliquity of the bisectrix. At any rate the result is smaller 

 than is usual for augite. This mineral is also considerably decayed, and the product 

 of such change seems to be, as remarked by Wadsworth,i a dirty green fibrous 

 product. 



Quartz is found in a mass of decomposition products resulting, according to 

 Wadsworth, from alteration of the groundmass. It also forms a micro-pegmatyte in 

 in the feldspars. Three sections examined. 



Age. Cabotian ; Beaver Bay diabase. 



liniinrk. The writer formerly identified the foregoing described augite as 

 hypersthene, and gave to the rock the name hyperyte, but the mineral is no doubt 

 the same that occurs usually in this connection. 



Prof. W. S. Bayley has assumed, probably from the geographic position of this 

 rock (Journal of Geology, vol. ii, page 819), that it is no part of the gabbro, and that 



*This instrument is described in the Aim-rii-mi ili-nlniiisi, vol. xvii, p. 79 (1896). 



t M inr:i-aux des Roches, p. 365. 



J Bulletin Hi, Geological Natural History Survey of Minnesota, p. 76. 



