54 CALIFORNIA ACADEMY OF SCIENCES. LPROC. 30 SER. 



sively the relations of the breccia to the main body of the 

 porphyrite and to the andesite, but some of the evidence 

 points to the probability of its being older than either. 



E. BASEMENT SERIES. 



In surface area the rocks of the basement series cover a 

 little more than half the island. They consist mainly of 

 quartzites and mica -schists, with several smaller areas of 

 talc- and amphibole-schists and serpentine. 



The occurrences of the other basement rocks within the 

 quartzite area were not exactly mapped, and for that reason 

 they appear on the map without definite boundaries. The 

 actinolite areas, in particular, are more extensive than is 

 here indicated. The main occurrences of these rocks are as 

 indicated, but smaller areas of all of them occur elsewhere 

 within the quartzite area. The actinolite-schists, besides 

 their main occurrence, are found in the area of the quartzite 

 about Middle Ranch Canon. The areas of the basement 

 rocks found within the porphyrite area northwest of Avalon 

 are in part of actinolite- and talc-schists, with some ser- 

 pentine. Talc-schist is also found near the center of the 

 west end, toward the northern coast. Besides the ser- 

 pentine areas mapped, a small patch was found on the ridge 

 to the south of Middle Ranch Canon, and another not far 

 from the extreme northwestern point of the island. A 

 patch of garnet amphibolite is found just to the west of the 

 border of the andesite in the Little Harbor region. 



I. QUARTZITE. 



The quartzite occurs distinctly bedded, and wherever it 

 was possible observations for dip and strike were made, 

 though these were insufficient to warrant a statement with 

 regard to the beds as a whole. In many places these read- 

 ings give no real indication of the general dip and strike of 

 the series, owing to local folding with minor plications, and 

 to occasional faulting. By a comparison of the various 

 readings made, however, the western division of the island 



