Ransome. I 



Eruptive Rocks of Point Bonita. 



99 



associated with the diabase, about half way between B and C, shows 

 a compact gray rock of cryptocrystalline texture, which, under the 

 microscope, is seen to be composed of a groundmass of lath-shaped 

 plagioclases and unindividualized glass, in which are scattered a 

 few large feldspathic phenocrysts. Both the lath-shaped and por- 

 phyritic crystals are unusually fresh, and give extinction angles 

 ranging from 23 to 28°, the most common angle being about 25 . 

 This would indicate their position to be in the labradorite series. 

 Opaque iron ores are present in the form of minute specks, and 

 there are a few small round amygdules of calcite and chlorite. A 

 chemical analysis of this rock is given in column IV of the table. 



A fragment from the exterior of a similar spheroid at C. is iden- 

 tical in macroscopic appearance, but shows a more glassy structure 

 under the microscope. In addition to the ordinary lath-shaped 

 plagioclases, which are often of unusual length, there also occur 

 smaller individuals which are aggregated into open radial and brush- 

 like groups, which may be regarded as imperfect spherulitic growths.* 

 They are more perfect examples of a similar structure already 

 described in connection with the spheroidal basalt. Similar radial 

 groupings of the feldspars have been noted by Miss Raisin f in the 

 spheroidal basalt of the Lleyn, and by Cole and Gregory % at Mont 

 Genevre. 



Variolitic Fades. — The very constant association which appears 

 to exist in European localities between spheroidal diabase or basalt 

 and variolitic structure, led to a careful search for varioles in the 

 more compact phases of the rock under investigation. The effort 

 met with success, although the varioles were found only in one 

 spot, and are certainly not a constant concomitant of the spheroidal 

 forms, as they were not detected at all in the lower basalt. About 

 midway between C and D, near the base of the cliff, and therefore 

 at some depth within the eruptive mass, small round bodies were 

 observed standing out from the weathered surface of an imbedded 

 spheroid. They were of the same gray color as the rock, which, 



*Iddings, Spherulitic Crystallization, Bull. Phil. Soc. Wash., Vol. XI, p. 459. 

 f Loc. cit., p. 153. 



JThe Variolitic Rocks of Mont Genevre, Q. J. G. S., Vol. XL VI, p. 313, 

 Plate 13, Fig. 5. 



