Ransome-1 



Eruptive Rocks of Point Bonita. 



79 



Figure 3. — Diagrammatic sketch of cliff at F, showing spheroidal basalt over- 

 lain by pyroclastic rock. The upper portion of the cliff is dia- 

 base, which can also be seen intrusive into the basalt below. 



geneous solid, but is composed of separate, closely-fitting bodies, 

 of various sizes, up to about two feet in diameter, and of such shapes 

 that their sections show circular, elliptical, and other less regular 

 forms, bounded by closed curves. The bounding surfaces are per- 

 fectly sharp and definite, and entirely inclose each separate sphe- 

 roid, nothing resembling unclosed shells or spirals, or otherwise sug- 

 gesting "perlitic structure on the large scale,"* being discernible. 

 There are no empty spaces between these spheroids, but they mutu- 

 ally conform to each other's shape, as if each had been moulded 

 for the place it fills. Generally the adjacent bodies are separated by 

 a secondary layer of impure calcite, whose thickness seems entirely 

 independent of the size of the spheroids it separates. The sphe- 

 roidal rock is here overlain by a bed of pyroclastic rock, which will 

 again be referred to. 



The last occurrence of this spheroidal rock is at B, where it also 



* G. H. Williams, The Greenstone Schist Areas of the Menominee and 

 Marquette Regions of Michigan, Bull. U. S. G. Surv. , No. 62, p. 167. 



