264 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



i<\ Plagioclase laths are seen distributed in a background which is composed 

 of auqite in small grains, magnetite, chloritic alteration product, and dirty, faintly 

 brownish areas. These areas (which are abundant), except for small bright points, 

 show no effect on polarized light, and they are thus referred to an originally glassy 

 condition of the rock. The large amount of this glassy material is noticeable. The 

 black blotches noticed on the hand sample are seen to be pseudamygdaloidal aggre- 

 gations of the chloritic material. 



One thick section. 



Age. Cabotian. u. s. G. 



Remark. In the field this dark-gray rock is seen to be partly brown, and in some 

 places it is about half brown. It is thus shown to have outwardly, as it has micro- 

 scopically, a close relation to the rocks Nos. 217-219, in association with which it 

 appears along the beach. N. H. w. 



No. 221. DIABASE. (Coarse.) 



At a distance of about twenty rods east of the mouth of Brule river. 



Ref. Annual Report, ix, page 56; Annual Report, x, pages 43, 140; Bulletin ii, pages 76, 100, 101; American 

 Association for the Advancement of Science, vol. xxx, page 163. 



Meg. A coarse rock, rather dark colored and easily rusting. Evidently one of 

 the most recent eruptives of the region, judging by its contents and from the general 

 structural relations. The crystals are all fresh and of large sizes. 



Mic. The feldspar is twinned, both on the pericline and albite plans, the 

 lamellae crossing at varying angles, as cut by the plane of the section. It is very 

 fresh and includes fine needles, apparently of apatite. The highest extinction angles 

 on 010 range from 25 to 27, indicating labradorite. Several sections show a single 

 optic axis in convergent light, the figure consisting of a single straight bar, which 

 rotates in a direction the reverse of that of the rotating stage. Such sections are 

 neither dark nor colored, but show a dim light, and are thus readily recognized in a 

 thick section like the one examined, whose thickness, judging from the colors of 

 the labradorite, compared with Michel Levy's colored scale (Determination des Feld- 

 spaths, plate), is about .07 millimeters. The colored fringes on the augites indicate 

 the same thickness. When the albite and pericline bands are at or near right angles, 

 as happens sometimes when the crystal is cut in the zone 001:100, the narrow sharp 

 pericline striae maintain their integrity through the albite bands. The section also 

 shows Carlsbad twinning, the twins themselves being also twinned. 



Wadsworth mentions orthoclase and quartz in this preparation, but we can find 

 no orthoclase. There is a little quartz in the altered magmatic remnants. 



The augite is well preserved, the thickness of the section not producing any 

 dimness in the colors or in the transparency. It is coarsely fissured, but does not 

 seem to pass into diallage. It exhibits a colored striation or banding, occasionally 



