Ransome.] 



Geology of Angel Island. 



of this glassy facies is decidedly high, being 3.08, whereas the basalt 

 glass of Some which is mentioned by Judd and Cole* as being excep- 

 tionally high, is only 2 89. As the specific gravity of the fourchite 

 is 3.20, the difference between its specific gravity and that of the 

 glassy facies is not greater than that often found between a rock- in 

 its crystalline ami its glassy conditions. 



A brecciated facies has previously been referred to as occurring 

 in the cliffs along the beach just south of the military barracks. 

 This consists of small fragments of eruptive rock, generally rather 

 angular in shape, and held together in a matrix of still smaller par- 

 ticles. Inclusions of chert are quite frequent, and generally bear 

 evidence of having experienced some mechanical force, and exhibit 

 a certain amount of alteration. A similar breccia also occurs in 

 smaller amount near the end of Point Stuart. In both cases these 

 breccias are plainly contemporaneous facies oi the intrusive four- 

 chite with which they occur, and a subaerial origin for them is put 

 out of the question by the fact that, both near Point Stuart and 

 Point Knox, the whole mass of the eruptive rock of which they are 

 a part is capped by considerable masses of chert that have been 

 heaved up and altered by the intrusive igneous mass. Moreov er, 

 near the small chapel which stands upon the hillside south of the 

 military post, a small dyke, coming from the same eruptive bod)' of 

 which the breccia is a part, can be seen cutting through the sheared 

 and disturbed sandstone. 



It thus appears that, under certain circumstances, of which it is 

 rather difficult to form a mental picture, a rock may be intruded in 

 a brecciated condition, and the same statement holds, of course, in 

 regard to the spheroidal facies, with which the breccia is most 

 closely associated, and which has previously been shown to be com- 

 patible with intrusion at Point Blunt. Itis incredible, however, that 

 such rocks could have been intruded at any considerable depth. 

 They cannot have been subjected when in a molten or plastic state 

 to the pressure of thick overlying strata, and it is equally clear that 

 they must have cooled with comparative rapidity. It appears 

 highly probable that they represent material that has been sub- 



*On the Basalt-glass (Tachylyte) of the Western Isles of Scotland, O. J. G. S.> 

 Vol. XXXIX, 1883, p. 450. 



