88 



University of California. 



[Vol. i. 



also been considerable movement and disturbance posterior to the 

 eruption of the diabase, is evidenced by the crushing and calcifica- 

 tion which the latter has undergone, particularly north of E. 



A small exposure of eruptive rock occurs at M, on the south 

 shore of the lagoon, but is too much decomposed to identify. It 

 appears to be intrusive, and probably belongs to the diabase. 



On the eastern border of the map a single point of eruptive 

 rock was observed projecting from the soil, only a few feet from the 

 sandstone, from which it is probably separated by a fault. Whether 

 it belongs with the spheroidal basalt or with the ordinary diabase 

 is not determinable within the limits of the map, and the question 

 is of no immediate importance. It probably is a portion of the lat- 

 ter, and is so mapped. 



Microscopic Petrography. — In typical hand specimens the diabase 

 is a dark gray-green rock, with distinct crystalline structure, in 

 which feldspars and a dark mineral, presumably pyroxene, can be 

 distinguished with the naked eye. Under the microscope it gen- 

 erally shows a holocrystalline, coarsely ophitic structure, made up 

 of more or less idiomorphic plagioclase, with the pyroxene filling 

 the interstices in large irregular plates, often optically continuous 

 over a large portion of the slide. Iddingsite * is frequently present, 

 as an important constituent, in large, rounded, idiomorphic forms. 

 Opaque iron ores and apatite are usually present, but vary much 

 in abundance in different specimens. Following German and Amer- 

 ican usage the rock would probably be classed as a diabase. 



The feldspars are, as a rule, too much kaolinized in the more 

 coarsely crystalline specimens to allow of measuring the extinction 

 angles, as only the traces of polysynthetic lamellae remain; but in 

 some of the finer-grained phases the plagioclases are fresher, and 

 give extinction angles, averaging, in different slides', from 23 to 32 

 on either side of the twinning plane, which would place it in the 

 labradorite series. It appears very probable that not all the plagio- 

 clases are of the same species. 



Augite, the only pyroxene present, occurs in large plates and 

 in irregular grains, there being rarely any traces of undoubted 



*See "Geology of Carmelo Bay," by Andrew C. Lawson, this Bulletin, 

 Vol. I, pp. 31-36. 



