MAEYLAND GEOLOGICAL SURVEY 141 



face is a reddish yellow. The rock readily weathers into cubical 

 blocks possessing this oxidized crust, which penetrates to variable 

 depths, and encloses a dark green or gray center. 



The two essential constituents of the diabase are present in the 

 usual proportion and exhibit characteristic diabasic structure. 



There are two varieties of pyroxene, a violet colored augite and 

 a colorless monoclinic pyroxene, which is both more idiomorphic and 

 more altered than the augite. It resembles salite, but differs from 

 this variety of pyroxene in the possession of a smaller optical angle. 

 The narrowly lath-shaped feldspar is a basic labradorite (Ab 2 An 3 — 

 Ab 3 An 2 ). Magnetite and apatite are accessory constituents. The 

 constituents are remarkably fresh. The pyroxene shows some slight 

 alteration to green hornblende, but the more conspicuous alteration 

 product is a yellowish chlorite which exhibits the double refraction 

 and other characters of delessite. This more often replaces the color- 

 less pyroxene, but is also an alteration product of the augite and of 

 the labradorite. Calcite is a very minor alteration product. 



The rock does not vary in any essential respect from the Triassic 

 diabase of West Rock, New Haven, which has been described 1 in 

 detail by Professor Pirsson. 



CHEMICAL COMPOSITION. 



I. II. 



Si0 2 



50.79 





52.53 



A1 2 3 



14.19 



14.35 



Fe 2 3 



3.84 



5.93 



FeO 



7.44 



5.45 



MgO 



7.88 



7.99 



CaO 



9.75 





10.27 



Na 2 



1.89 





1.87 



K 2 



.95 





.92 



Ignition 



1.95 





1.23 



Ti0 2 



70 





.32 



P o 0\ 



.15 





.15 



MnO 



48 





trace 









s 



.08 





100.01 



Cu 



trace 







Li 2 .. 



. . faintest trace 



101.04 

 I. — Diabase dike at Williamson's Point, Lancaster Co., Pa. 

 II. — Diabase from York Co., Pa. 



Analysis made by Dr. F. A. Genth, Jr., Penn. 2nd Geol. Surv. Vols. CCC, p. 275, 

 and C, p. 123. 



1 Bulletin No. 150, IT. S. Geol. Surv., pp. 264-273. 



