Ransome.] 



Eruptive Rocks of Point Bonita. 



103 



spheroidal basalt. Its occurrence in well-developed crystals in a 

 rock otherwise showing only microlitic forms, strongly emphasizes, 

 moreover, the fact that it was an early separation from the magma. 



The varioles found, were too few in number to throw any new 

 light upon the relation existing between their occurrence and 

 spheroidal structure. There appears to the writer, however, no 

 good reason for considering them in this case as due to secondary 

 devitrification,* but they seem rather to owe their origin to some 

 physical circumstances f attendant upon the original cooling of the 

 magma, Miss Raisin having come to a similar conclusion in regard 

 to the variolite of the Lleyn.J Nothing like the perlitic cracks of a 

 devitrified tachylyte have been detected, and the variolitic structure 

 must apparently be regarded as closely related to the radial group- 

 ings of plagioclase previously described, which are certainly not 

 secondary in origin; indeed, Loewinson-Lessing§ has shown that 

 the one structure can be traced directly into the other. 



With the exception of the very doubtful occurrence described 

 by G. H. Williams, || the writer is not aware of any description of 

 variolite from an American locality. 



THE PYROCLASTIC ROCKS. 



These have been found in only two places in the area, near the 

 lighthouse and at F. At the former place it occurs as a rather 

 thin, irregular, and interrupted band, running diagonally down to 

 the water's edge on the south side of the cliff. (See section B' B".) 

 It consists of rounded and angular fragments of very vesicular rock, 

 cryptocrystalline in texture, and resembling the rock of the sphe- 

 roids. These fragments average two to three inches in diameter, 



*G. A. J. Cole, On Some Additional Occurrences of Tachylyte, Q. J. G. S., 

 Vol. XI. IV, p. 300. 



Also, Cole and Gregory, The Variolitic Rocks of Mont Genevre, Q. J. G. 

 S., Vol. XLVI, p. 314, 1890. 



t F. Lcewinson-Lessing, Die Variolite von Jalguba im Gouvernement Olenez, 

 Tscherm. Min. u. Petr. Mitth, B. 6 N. F., pp. 299-300, 1885. 



% Loc. cit., p. 297. 



I Loc. cit., p. 156. 



\\Loc. C7t.,p 173. 



