PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 161 



Scoria. Diabase.] 



Chemical analysis. This analysis was made by Prof. J. A. Dodge and first published in the Thirteenth 

 Annual Report, page 100 (No. 153): 



SiO 2 65.50 



A1 2 O, 10.0C 



Fe 2 O 3 14.40 



PeO .23 



CaO .96 



MgO .73 



K 2 2.88 



Xa 2 O 2.lT> 



H 2 O .80 



Total 97.93 



A i/i'. Cabotian. u. s. G. 



No. 75. SCORIA. (Acid, volcanic.) 



East of Lester river. S. E. % sec - 34, T. 51-13. At this locality there has been an unusual disturbance, 

 the strata dipping in different directions, or at a high angle to the north, the strike being nearly east and west. 

 This point cannot be far distant from an ancient volcanic crater. (See figure 109, page 570, vol. iv.) 



Ref. Annual Report, ix, page 22. 



Meg. The rock varies from a brownish red amygdaloid to a flesh-red scoria, 

 the latter represented by the samples examined. This is porous, and even spongy 

 but the vesicles are elongated and flattened. Through the rock runs a considerable 

 calcite in veins and patches. 



Mic. The sections show that the rock consists essentially of devitrified glass 

 and of glass, embracing vesicular openings. There are many crystallites which are 

 apparently biaxial, and others evidently of quartz though they do not exhibit 

 hexagonal shapes. They are clear and quick to extinguish between the nicols. But 

 these are embraced, frequently, in a substance which is nearly isotropic. Yet, 

 between the nicols other faintly luminous spots appear, indicating the arrested 

 formation of other crystallites. In other words, the glass is but imperfectly devit- 

 ritied. This fact is in keeping with numerous observations on the tuffaceous rocks 

 of the region, in which sub-glassy grains have been seen distributed through a 

 clastic mass. 



Two sections. 



Ai/t: Cabotian. N. H. w. 



No. 76. DIABASE (ivith divine an.<J l/n/lite). 



Directly overlying No. 75, interbedded in No. 75, but still runs under No. 77. 

 Kef. Annual Report, ix, page 22. Compare No. 91B . 



Has a trappous aspect, contains laumontite amygdules and nodules of 

 calcite and fluorite. 



Mic. The aspect is that of an ophitic rock, the bright microlites of felil^xn- 

 being numerously embraced in some of the large (/i/(/ifi>x. There is no evidence of 

 an earlier generation of augite. 



The of in' iir is much crowded with magnetite accumulations, and in many grains 

 is so lost that its form is the only remaining evidence of its former existence. 

 12 



