BASIC VOLGANIOS OF HEMLOCK FOEMATIOI^. 



113 



basalt is very dense, with only occasionally small clilorite-filled vesicles in 

 it, and there is uo true flow structure observable. The facts cited seem to 

 show that this ellipsoidal portion was the surface of a lava flow, whether 

 the top or the bottom is immaterial. In certain cases the ellipsoidal facies 

 may constitute an entire flow. Where the direction, of flow could with any 

 degree of certainty be determined, it was seen that the two longer axes of 

 the ellipsoids are in the plane of the flow. 



The ellipsoids vary in size from a few inches to 6 or 8 feet in diameter, 

 and are usually spoken of as spheroids. Attention has already been called 

 to the incorrect usage of this term by F. Leslie Ransome, in his interesting- 

 paper on "The eruptive rocks 

 of Point Bonita, California."^ 

 The outlines of the bodies are 

 circular onlv in exceptional 

 cases. On the other hand, 

 sections in all directions 

 through them give almost in- 

 variablv ellipses, and there- 

 fore they are more properly 

 ellipsoids than spheroids. On 

 the surfaces exposed the long 

 axes of the ellipses lie in the 

 same general direction. 



The ellipsoids are formed 

 of a very fine-grained porphy- 

 ritic or nonporphyritic rock. 

 This is amygdaloidal or non- 

 amygdaloidal. Where amygdaloidal, the amygdules are as a rule dis- 

 tributed throughout the ellipsoids, though on the whole the masses are more 

 scoriaceous on the periphery than near the center. In exceptional cases, 

 the amygdules are much more numerous on the west side of the ellipsoids 

 than on the east side (fig. 8). In such cases the west sides are toward the 

 tops of the lava flows. The ellipsoids are very commonly split up by 

 cracks. Some of them have a roughly radiate arrangement. These may be 

 due to the effects of contraction in the early stao-es of the existence of the 



Fig 8 — bketch shon mg the conceDtration of the amygdaloidal cavities 

 ou mie aide ol .m (.UiDSoid this side probably representing the side 

 niau-,t the ■,uitic6ot tlie How 



' Op. cit., p. 75. 



MON XXXVI- 



