S6 HAEHL AND ARNOLD — THE MIOCENE DIABASE. [Feb. 5, 



ered. The series of beds in which the dikes occur north of the 

 Langley house have an upward sequence of sandstone, tuff, limy 

 shales, and then alternating thick beds of tuff, comparatively thin 

 beds of limestone and limy tuff, the whole capped by sandy tuff, 

 above which are shale and sandstone beds (see Fig. 14.) Soon 

 after the deposition of this series, and before the tuffs and lime- 

 stones had become very coherent, diabase was intruded between 

 the lower sandstone layer and the overlying tuffs. The intruded 

 bed fractured the tuff along lines approximately perpendicular to 

 the bedding planes of the series, and the unconsolidated ooze and 

 limy tuff of the interbedded layers flowed into the fissures, thus 

 forming the dikes. 



T/i£ Diabase. 



Studied in the field the diabase presents two facies. One will be 

 termed the diabasic, the other the basaltic. The distinction is 

 made purely on the physical appearance of the two. No great 

 chemical difference exists, but the crystallization, color^ texture and 

 manner of weathering are so radically different that, while no dif- 

 ferentiation is attempted on the map, a distinction is necessary in 

 describing the rocks microscopically. Secondary dikes of small 

 proportions were found in the diabasic type, and will be briefly 

 described under that head. 



The diabasic facies. — The diabasic type seems to be confined to 

 the masses which make the north and east boundaries of the area 

 between the south fork of Tunitas Creek on the north and Langley 

 Hill on the south. In all cases it lies along the crest of the range, 

 making the highest peaks and giving them a peculiar rounded out- 

 line that is readily distinguishable at a distance. The rock is well 

 exposed near the summit of the ridge, on the road which crosses 

 the range two and one-half miles south of Sierra Morena. Here 

 the course of the dike is plainly marked by the large rounded 

 boulders on the hillsides. The rocks weather in such a way here 

 as to give particular prominence to the feldspars, thus giving the 

 mass the appearance of a gabbro. The soil derived from its disin- 

 tegration closely resembles granitic soil. It is made up of granular 

 particles with a slight reddish cast, and varying in size from a 

 diameter of one quarter to one-sixteenth of an inch. 



Macroscopically the rock is a medium grained, light gray, crys- 

 talline aggregate, in which three components are very readily dis- 



