Ransome.] 



Eruptive Rocks of Point Bonita. 



75 



THE SPHEROIDAL* BASALT. 



The eruptive rocks admit of a threefold division, — the sphe- 

 roidal basalt, the diabase, and the associated pyroclastic rocks. 

 These will be described in the order named. The difficulty experi- 

 enced in defining with precision the boundary between the first two 

 rocks, led to the collection of a rather numerous suite of specimens, 

 considering the smallness of the area, and the preparation of about 

 sixty microscopic slides, thus affording sufficient material for a full 

 petrographical description. 



Occurrence and Structural Peculiarities. — Immediately south of 

 the narrow neck of rock joining the fog signal and lighthouse, and 

 close to the latter, a rocky mass juts into the ocean, and is connected 

 with the cliff, some fifty feet down, by a natural bridge of rock. Its 

 appearance, seen from a position west of the lighthouse, is well rep- 

 resented in Plate 7, Fig. I, and the illustration shows very strikingly 

 one phase of the remarkable structure which characterizes the 

 spheroidal basalt, and suggests its name. Upon clambering down 

 to the rock, it is seen that the curved figures shown in the illustra- 

 tion are the transverse sections of elongated, bolster-like, or bale- 

 like masses, whose longer axes are in general roughly parallel and 

 extend downwards and away from the observer, that is, dip in an 

 easterly direction. An examination of the cross sections of these 

 bales shows that their material is not quite uniform, they frequently 

 exhibiting a tendency to weather out near the center, leaving a sur- 

 rounding shell, as may be seen in the upper left-hand corner of the 

 figure; moreover, although the whole rock is full of amygdules of 

 dark green color, these seem larger, and more numerous, in the 

 interior portions of each bale, and particularly near the weathered 

 inside surface of the surrounding shell. The masses are also much 

 fractured transversely, each crack being usually accompanied by 

 miniature faulting. Some amygdules were observed filling greatly 

 elongated vesicles. 



Upon looking back at the cliff, the line of contact between the 



*The adjective "spheroidal" is used throughout this paper in deference to 

 previous usage only; for, as will appear in the sequel, the forms assumed by 

 the basalt can seldom properly be called spheroids. 



