Lawson ) 



Geology of Carmclo Bay. 



>7 



which the Santa Lucia granite has been subjected. An instance 

 of this is shown in the photograph of a cliff face on Malpaso Canon, 

 a little to the south of Carmelo Bay, which is reproduced in Plate 

 2, Fig. I. 



Pegmatite Dykes. — Beside the dykes of granite there are numer- 

 ous narrow dykes of pegmatite which also traverse the Santa Lucia 

 granite. These dykes are usually only a few inches in width. 

 They consist essentially of a very coarse granular aggregate of 

 orthoclase and quartz, with occasionally some black mica in broad 

 plates and a few shreds of muscovite. The feldspar very commonly 

 has a pronounced flesh tint, and is fresh and lustrous, while in other 

 cases it is kaolinized and bleached white. In a few cases the cleav- 

 age surfaces of the feldspar show a "luster mottling" similar to that 

 characteristic of the phenocrysts of the Santa Lucia granite; but 

 this does not appear to be a common feature. 



The age of the pegmatite dykes relatively to the granite (aplite) 

 dykes has not been determined, as they have not been observed to 

 intersect. Possibly both aplite and pegmatite are of synchronous 

 formation. 



Relation to Other Formations. — After the intersection of the 

 Santa Lucia granite by dykes of granite (aplite) and pegmatite, the 

 mass was deeply eroded. The coarsely crystalline granular charac- 

 ter of the rock is warrant for believing that it solidified under deep- 

 seated conditions, and that its plutonic facies has been exposed Only 

 by the removal of a large volume of superincumbent rock. The 

 eroded surface of the granite, where it passes beneath the sedimen- 

 tary rocks, is well seen in cliff sections, particularly at Carmelo 

 Point, where a photograph of the contact was obtained, which is 

 reproduced in Plate 3. The surface of the granite here passes beneath 

 a basal conglomerate of the Carmelo series. The surface is smooth 

 and fresh, and presents no evidence of important secular decay prior 

 to the deposition of the conglomerate. The surface is much 

 smoother than the surfaces presented by the granite where not 

 protected by an overlying formation. The difference may perhaps 

 be ascribed to the progress of secular decay since the deposi- 

 tion of the conglomerate, the tendency in a partially decayed rock 

 being to weather rougher than in the case of a fresh rock. But 



