Ransome ] 



Geology of Angel Island. 



All the chert resting upon the fourchite to the west of the ser- 

 pentine, has been more or less altered. That least affected is on 

 the hill east of Point Stuart, about in the center of the mass. 

 Pieces were collected here hav ing a light yellowish-gray color, with 

 a rather schistose fracture, and show ing a very fine granular texture 

 with no particular luster. Under the microscope it shows evidence 

 of pressure and shearing. It is composed of little spherical bodies 

 of comparatively clear cryptocrystalline silica, which in size and 

 shape exactly resemble the radiolarian remains ahead}' described, 

 lying in a darker less transparent matrix. The conclusion is easy 

 and natural, that this rock represents a chert which, originally rich 

 in radiolaria, has been subjected to intense pressure. The little 

 spherules of pure silica being more resistant than the inclosing 

 matrix, have been crowded together, while still preserving their form, 

 and the matrix has been forced to recrystallize in the shearing planes. 

 Under high powers, the cloudiness of the matrix is seen to be due to 

 a thick dusting of minute crystal needles and granules, but they are 

 much too minute for identification. 



Another specimen, taken near the former, has a thoroughly 

 jaspery look and a splinter)' fracture. Under the microscope, and 

 with high power, it is seen to be composed of closely fitting grains 

 of recrystallized quartz, through which are strewn microlites of 

 nearly colorless amphibole, in slender, acicular forms, and also, what 

 is apparently the same mineral, in short, stout prisms without crys- 

 tallographic terminations, and having a light yellowish tint. The 

 stouter microlites have a tendency to cluster together, forming 

 nuclei from which the longer needles project. Other microlites 

 and grains appear to be present, but were not identified. 



Upon turning to the map, it will be seen that the cherts, which 

 crown the hill of Point Stuart, run down to the beach on the north 

 as a narrow ledge, the bedding" planes being tilted up at a steep and 

 variable angle. Immediately at the beach, the fourchite is very 

 clearly intrusive into the cherts, penetrating" the latter irregularly 

 and being full of detached inclusions. The chert bears evidence of 

 strong crushing and subsequent resilicification and veining. Close 

 to the contact the chert has taken on the characteristic blue tint 

 that invariably serves as an indication of the presence of glauco- 



