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University of California. 



[Vol. i. 



three hundred yards in width. The linear arrangement of these out- 

 crops as they appear on the map was strongly suggestive of a dyke 

 or sill, and the petrographical character of the rock, which proved to 

 be an altered hypersthene diabase, made its intrusive origin seem 

 certain. The outcrops, however, were found to be in every case of 

 very limited extent, never more than one hundred feet in greatest 

 dimension, and each completely surrounded by serpentine. The 

 explanation of this rather contradictory evidence was found in a 

 quarry section on Potrero avenue. On the face of the high bank 

 there exposed may be seen a series of slightly inclined, lens-shaped 

 masses of the intrusive rock, each surrounded by serpentine, and 

 separated from the neighboring masses by both horizontal and ver- 

 tical intervals of varying amount. One of these lenses, which was 

 in perfect contact with more or less slickensided serpentine 

 throughout its circumference, measured twenty-five feet in length 

 and five feet in greatest thickness. The rock showed no evidence 

 of crushing or shearing, but the lens was rounded off and the ser- 

 pentine moulded to it very much as in the case of the spheroids of 

 lherzolite before described. It is thus evident that the intrusive rock 

 once existed as one or mure continuous masses of the form of dykes 

 or sills, which were broken up into the present detached masses by 

 movements of the serpentine. This view of the nature of these 

 masses is confirmed by the fact that the texture of the rock varies 

 with the distance from the contact of the serpentine. In several 

 places where the full width of the dyke was exposed, a definite 

 increase in the coarseness ©f grain of the rock could be observed 

 as the center of the mass was approached. Whether this well- 

 recognized feature of dyke structure is accompanied by variations 

 in chemical character has not yet been ascertained. 



Petrographical Character. — The intrusive rock is normally a 

 massive rock, dark gray to whitish gray in color, presenting in all 

 cases a distinctively crystalline aspect; it is hard and brittle and 

 rings when struck with the hammer. In places a tendency towards 

 a schistose structure is observed, plainly a result of the shear- 

 ing forces which have been shown to have affected the rock; but 

 this structure is a very local feature. In hand specimens the re- 

 cognizable minerals are feldspar, generally milky and opaque, horn- 



