220 



University of California. 



[Vol. r. 



the sandstone, but the southwesterly, or upper side, is defined by a 

 line of greater complexity ; moreover, several smaller outcrops occur 

 on this side, which are superficially isolated from the main dyke. 



Petrographic Character. — Macroscopically, the serpentine pos- 

 sesses peculiarities which the writer has not observed elsewhere, — 

 not even in the serpentine at Tiburon, on the other side of Raccoon 

 Strait, which is apparently continuous with that of Angel Island. 

 A weathered block resembles at first glance a coarse conglomerate, 

 although pebbles and matrix appear of the same color. Closer 

 inspection shows at once that the whole rock is serpentine, and is 

 made up of hard, compact nodules, ranging in size from an inch to 

 a foot in diameter, imbedded in a softer, much sheared matrix. 

 When one of the nodules is broken, it is seen to be composed of a 

 very pure, homogeneous serpentine, green-gray in color, translucent, 

 and breaking with a splintery fracture. The texture appears to be 

 very fine saccharoidal or granular, to compact. Small black specks 

 of magnetite and chromic iron are scattered sparingly through the 

 mass. The lens shows generally no trace of crystals, beyond here 

 and there a glistening needle or fiber. The cause of this nodular 

 structure is very clearly internal movements, whereby the whole 

 serpentine mass has been sheared and slickensided throughout, the 

 harder portions remaining as rounded and polished nodules. A 

 serpentine showing a somewhat similar nodular structure has been 

 described by Palache* at the Potrero, San Francisco. The sphe- 

 roids in the latter rock are larger than those at Angel Island, the 

 matrix is more crumbling, and the serpentine itself is somewhat 

 different. There can be no doubt, however, that the origin of the 

 structure is the same in both occurrences. One can hardly study 

 the serpentinized dyke upon the. island without being impressed by 

 the great discrepancy between the amount of internal movement 

 shown by its present structure, and the extent to which similar 

 shearing can be proved to have affected the adjacent rocks. This 

 would seem to suggest that the increase of bulk brought about by 

 the serpentinization of the original dyke rock may have been an 

 active cause of the pressure and shearing. 



"The Lherzolite-Serpentine and Associated Rocks of the Potrero, San 

 Francisco. This Bulletin, Vol. I, No. 5, p. 164 



