Ransome. I 



Geology of A tigel Island. 



'95 



The curved belt of eruptive rock (fourchite) shown on the map 

 is the exposed edge of an intrusive sheet or sill, that has followed 

 very roughly the general bedding of the sandstone, and has given 

 rise to more or less continuous zones of locally intense contact 

 metamorphism, adjacent to its upper and lower surfaces. On the 

 western side of the island there also occur extensive exposures of 

 eruptive rock, which is in part identical with that just mentioned, 

 but which passes also by insensible gradations into glassy and feld- 

 spathic facies, and even into a breccia, showing that it was erupted 

 at no great depth below the then existing surface. As would be 

 expected from this latter fact, the contact metamorphism is in this 

 case not so pronounced, and is mainly confined to the cherts, which 

 seem to be peculiarly susceptible to such alteration. Small, sporadic 

 intrusions of eruptive rock occur in many places in the sandstone, 

 each producing more or less accompanying alteration. As these 

 small intrusive masses are petrographically identical with various 

 facies of the sill rock, it is believed that they were erupted contem- 

 poraneously with the latter, and are products of the same original 

 magma. 



The most conspicuous, and, in many respects, the most interest- 

 ing geologic feature of the island, is the large serpentized dyke that 

 cuts across its western half in a northwest and southeast direction. 



Its fantastically weathered outcrops, projecting from the soil- 

 covered hill slopes, form a striking feature in the landscape, and, 

 together with numerous loose blocks and boulders, define its course 

 as a broad gray band stretching over hill and dale. The fact that 

 this serpentine has plainly resulted from the alteration of a dyke of 

 basic igneous rock, lends a strong local interest to the occurrence 

 on account of the persistency with which the view that the greater 

 part of the serpentine of the Coast Ranges is metamorphosed 

 sediments, still retains its place in the literature, confusing and 

 obstructing the way toward a clear understanding of the geology 

 of this region. 



Of far more general interest than the preceding, however, is the 

 pronounced contact metamorphism that has been effected by the 

 rock of which the serpentine is a derivative, and by the fourchite, 

 upon the cherts and sandstones through which they forced their 



