62 HAEHL AND ARNOLD — THE MIOCENE DIABASE. [Feb. 5, 



to its origin as a product of their partial disintegration ; while its 

 presence filling angular cavities between crystals of feldspar would 

 indicate that it might have replaced whatever mineral originally 

 filled that area, for it is hardly possible to conceive of that area 

 being left vacant after the magma had cooled and crystallized. 

 Yet, were it conceded that nepheline did occupy these areas, it 

 would still be impossible to believe that it could furnish enough 

 soda for the patches included in the feldspars. This problem 

 remains unsolved. 



Titanium is present in some quantity, as shown by the analyses of 

 the rocks discussed in this paper. None, however, is listed from 

 those studied by Fairbanks. It is possible that no determination 

 of that element was attempted by him. The presence of such an 

 amount of titanium, however, suggests that the augite, which is of 

 the pinkish variety both in this occurrence and in those described 

 by Fairbanks, carries some titanium. Ilmenite, which is sparingly 

 present, probably accounts for the remainder. 



Summary and Conclusion. 



Within an area of about three hundred square miles, most of 

 which is shown on the accompanying map, there are exposed about 

 thirty-five square miles of diabase in the form of tuffs, dikes and 

 intrusive sheets. The tuffs are interbedded with lower Miocene 

 strata and overlain by probable middle Miocene shales. The ba- 

 saltic facies of the diabase is partly contemporaneous with the tuffs 

 and partly of later origin, while the diabase facies is intruded into 

 the tuffs and middle Miocene beds. The igneous rocks under dis- 

 cussion are therefore of lower and middle Miocene ages. 



The tuffs are composed of fragments of the basaltic facies, gen- 

 erally angular, but sometimes water-worn. The tuffs are interbed- 

 ded with limestones, sandstones and shales. Intrusions of lime- 

 stone derived from the interbedded limy layers have been forced 

 into fissures in the tuff. 



Petrographically the diabase is of two general types. One is a 

 light colored, granular rock which is found along the crest of the 

 range north of Langley Hill. The other is a darker, fine grained, 

 basaltic type with occasional phenocrysts of olivine and augite ; the 

 latter type makes up the remaining area. 



The rock is uniform in its chemical composition, which approxi- 



