I IO 



University of California. 



[Vol. i. 



noted by the writer at Tiburon, Marin County; at Port Harford, 

 San Luis Obispo County; and on the summit of the north peak of 

 Mount Diablo. It is noteworthy that in these widely separated 

 occurrences the rock is always associated with the red jaspers, and 

 with what is apparently the San Francisco sandstone. The Mount 

 Diablo rock is referred to by Turner* as apparently "a greatly 

 decomposed diabase," he making no mention of its spheroidal 

 structure, or of a beautiful feather grouping of plagioclases, which 

 it exhibits under the microscope. 



THE ORIGIN OF THE SPHEROIDAL STRUCTURE. 



Spheroidal structures, similar to those described in this paper, 

 have been noted by several writers in connection with aphanitic 

 diabase or basalt, and various theories have been advanced to 

 account for them. Without attempting an exhaustive historical 

 review, the following references may be said to have a close bearing 

 on the forms under discussion: — 



In 1875 Mallet | explained the occurrence of the spheroids in 

 many basalts as due to the weathering and exfoliation of "pointed 

 pieces" of basaltic prisms. Bonney, % in the following year, devel- 

 oped this idea, by showing that the tendency of a mass cooling 

 from several different centers would be to produce spheroids by 

 contraction. While the foregoing theories undoubtedly offer true 

 explanations of many occurrences, they are inadequate for the case 

 in hand. Both, starting from roughly cubical forms, require that 

 the resulting spheroids should be separated by an amount of mate- 

 rial vastly greater than that actually found between the Point 

 Bonita spheroids. In 1881 Bonney § referred briefly to a spheroidal 

 structure in an altered basalt of Porth-din-Lleyn, but offered no 



*The Geology of Mount Diablo, California, Bull. Geol. Soc. of Am., Vol. 

 2. P- 385- 



t Origin and Mechanism of Production of the Prismatic Structure of Basalt, 

 Phil. Mag., 4th Ser., Vol. 50, p. 220. 



J On Columnar, Fissile, and Spheroidal Structure, Q. J. G. S., Vol. XXXII, 

 p. 151, 1876. 



I On the So-called Serpentine of Porth-din-Lleyn, Q. J. G. S., Vol. XXXVII, 

 P- 50. 



