26 ALASKA GEOLOGY 



mainland, and to the southwest through the double line 

 of islands on the west side of Prince William Sound. It 

 is probable that they are older than, and occupy the axis 

 of an anticline in, the Vancouver Series. 



COOK INLET 



Mr. Palache landed at Halibut Cove, in Kachemak Bay, 

 Cook Inlet, and found an interesting section of green and 

 red radiolarian cherts, in thin and very regular beds sepa- 

 rated by clay or shale partings, beautifully folded and con- 

 torted (plate n). With them are associated intrusive masses 

 of diabase, much crushed and altered, showing in places a 

 distinct spheroidal structure, the surfaces of the spheroids 

 being largely covered with minute spherulites. Small 

 amounts of sandstone and conglomerate are also present. 



The series is cut by a group of conspicuous light-col- 

 ored porphyry dikes, standing nearly vertical, parallel, and 

 20, 10, 50 and 60 feet in width, respectively. Under the 

 microscope these dike rocks proved to be much altered 

 dacite-porphyries, showing phenocrysts of embayed 

 quartz, acid plagioclase much altered to calcite and kao- 

 lin, and occasional orthoclase in a granular to granophy- 

 ric groundmass of quartz and feldspar. Chlorite is spar- 

 ingly present throughout the rock, but the bisilicate from 

 which it was derived could not be determined. The 

 dikes are quite coarsely porphyritic near their centres, but 

 toward the contact with the cherts become almost aphan- 

 itic. The cherts are whitened for a few inches from the 

 contact but not otherwise altered. 



With the exception of the dike rocks, this section bears 

 an altogether extraordinary similarity in structure and 

 lithologic character to the radiolarian cherts and associ- 

 ated igneous and clastic rocks of the Franciscan Series 

 of the California Coast Range, especially well developed 

 on the San Francisco Peninsula. These have been de- 



