194 



University of California. 



[Vol. i. 



In shape, the island approximates somewhat roughly to an 

 equilateral triangle with sides about one and a quarter mile in 

 length, and embraces an area of about one square mile. The 

 northwest side, defined by Points Stuart and Campbell, forms the 

 southeast shore of Raccoon Strait. 



The topography, as the map shows, is comparatively simple, 

 the island having a central summit 771 feet* in height, from which 

 radiate the various spurs. Two of the latter, namely, those extend- 

 ing out to Point Camp- 



bell and to Point Tone, 

 dominate the others, and 

 form what might be 

 called a crescentic back- 

 bone to the island, — a 

 term the appropriateness 

 of which will be more 

 manifest when the geo- 

 logic structure is dis- 

 cussed. 



By far the larger por- 

 tion of the island is made 

 up of the San Francisco 

 sandstone, the general 

 disposition of the beds 

 being that of a synclinal 

 trough with its axis pitch- 

 ing approximately north- 

 west, so as to tilt the 















^ ^ 



(v 



it) ' 



^\ t — 





— \= 



i 



Pf 







; 

















trough toward the strait. Figure i— Index Map. 



A glance at the map 



shows that this structure is suggested topographically by the 

 crescent-like ridge just mentioned, and geologically by the almost 

 concentric band of eruptive rock presently to be described. 



Intimately associated with the sandstone are several small areas 

 of what in a former paper were termed "bedded jaspers," but which, 

 for reasons to be presented later, it seems preferable to designate by 

 the name of "radiolarian chert." 



"Pacific Coast Pilot, U. S. Coast and Geod. Surv., 4th ed., 1SS9, p. 183. 



