PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 171 



Diabase.] 



Hence the large cleavage plates are parallel to the clinopinacoid. The acicular 

 grains, which have a smaller extinction angle, are formed by the crossing of the two 

 cleavages 110 and 010. Compare Miti. des Roches, page 311. 



The rock in which this zeolite is embraced is an ophitic diabase considerably 

 decayed; evidently a continuation of No. 91. 



Four preparations (two sections). 



Remark. The number 92 is also applied to scoriaceous masses which are charged 

 with calcite and quartzine, and have but little of laumontite, obtained at the same 

 place. Layers of laumontitic and non-laumontitic rock follow each other in succes- 

 sion, making a zig-zag outline of the immediate coast, although its general direction 

 is nearly straight. The firm, massive layers, forming the points of the coast, were 

 the bottom portions of superficial flows of lava, and the vesicular layers, although 

 not always occurring with regularity, are the more superficial portions of the same 

 lava sheets. These alternations of hard lava-formed points and amygdaloid-lined 

 bays continue three or four miles eastward from Knife river, in some places, where 

 the bluffs rise thirty feet more or less above the water, several of the superimposed 

 layers being visible at once. About Agate bay this series is again exposed. 



N. H. w. 

 No. 93. DIABASE 



Point in sec. 10, T. 52-11 W. The bed, seven feet thick, forms the hight o5 the bluff, within a narrow bay, 

 but overlies a bed of six feet of very laumontitic amygdaloid, which is also brecciated; under that (nine feet to 

 the water) is a rock which has an outward resemblance to No. 91, but has less of thalite and more laumontite. 



Ref. Annual Report, ix, page 26. 



Mffj. A very fine-grained dark, sometimes brownish rock, containing a few 

 amygdules of chlorite, calcite and quartz. 



Mic. The slide shows minute lath-shaped feldspars enclosed in plates of augite. 

 Some ollr/i/e, much altered, is also present, and magnetite and hematite. The rock is 

 a good example of a very fine-grained " lustre-mottled " diabase, but it is so fine 

 grained that this lustre-mottling does not show megascopically. 



One section. 



Age. Cabotian. u. s. G. 



No. 93 A. DIABASE (with olivine,). 



Underlying No. 93. 



Ref. Annual Report, ix, page 26. 



Mrtj. A brown, fine-grained rock, containing much of a yellow mineral (thalite). 



Mir. Almost identical with No. 93. 



One section. 



Age. Cabotian. u. s. G. 



