Lawson. J 



Geology of Carmelo Bay 



9 



more compact and coherent fades resemble somewhat the rocks 

 of the Carmelo series, from which, however, they are easily discrim- 

 inated by the occurrence in them of fragments of white shale derived 

 from the Monterey series. These formations are spread over the 

 surface of the successive terraces which score the slopes of the hills 

 to an altitude of many hundred feet. They will therefore be referred 

 to in the following pages as the "Terrace formations." The same 

 designation is also used in the mapping to cover a terraced delta of 

 the San Jose, which is thought to be of Pliocene age. It is the newer 

 of these formations which cover the eruptive rocks and partially 

 obscure their relations to the older rocks. With the terrace forma- 

 tions are associated numerous Pholas borings in the rocks behind 

 the terraces. 



The only other formations which call for mention are the allu- 

 vial flood plain of the Carmelo River and the blown sand near the 

 shore. These formations will not be further noticed. 



THE SANTA LUCIA GRANITE. 



Petrography. — The Santa Lucia granite is a coarse-grained gray 

 rock with huge phenocrysts of orthoclase. The coarse granular 

 ground-mass of the rock consists essentially of vitreous quartz, 

 whitish or greenish white feldspar.and an abundance of black, lustrous 

 biotite. The quartz is the most largely developed mineral in the 

 ground-mass, there being areas of it as much as two centimeters in 

 diameter. The feldspar is less largely developed and the biotite 

 ranges in size from one to two millimeters. Striations maybe occa- 

 sionally detected on the basal sections of the feldspar in the ground- 

 mass, showing that some of it is plagioclase. 



The phenocrysts are the most striking feature of the rock. 

 These consist of immense crystals of glassy orthoclase, usually 

 elongated in the direction of the clino-axis. They are very com- 

 monly, but not always, twinned on the Carlsbad law. These pheno- 

 crysts not infrequently attain a length of ten centimeters, and from 

 four to five centimeters is a very common size. These elongated 

 crystals may be seen at favorable exposures, as in the quarry at 

 Carmelo Cove, to have sometimes a very pronounced parallelism 

 in their orientation in the ground-mass. Fig. i is a diagramatic 



