Lawson.] 



Geology of Carmelo Bay. 



41 



greenish yellow color, resembling an epidotic rock. In both fades 

 phenocrysts of idclingsite are abundant; and microscopic examina- 

 tion shows that there is no other phenocryst present. The idding- 

 site is heavily charged in most cases with iron oxide. In the green- 

 ish yellow variety the ground-mass is as usual chiefly a feltwork of 

 minute lath-shaped crystals of plagioclase with perhaps some inter- 

 stitial glass. In addition to this there are, however, considerable 

 quantities of a yellowish pleochroic mineral showing a longitudinal 

 monotomous cleavage (apparently fibrous in some instances) and 

 parallel extinction. It is the presence of this mineral which gives 

 the greenish yellow color to the rock. Its minute dimensions render 

 its determination difficult, but it is believed to be a later generation 

 of the iddingsite in the ground-mass. Magnetite is also present in 

 granules. 



In the blue variety of the rock the ground-mass is the same as 

 in the other, except that there are only a few scattered shreds of 

 the yellow, pleochroic mineral just mentioned. There are also 

 vesicular and amygdaloidal varieties, which are less dense in texture, 

 and are often limonitic. In addition to the facies above described, 

 there are at the Abalone Point area beds of volcanic agglomerate 

 and of breccia which have not been examined microscopically. 



The Lagoon Area. — At the north end of the bar, which, through- 

 out most of the year, shuts off the water of the Carmelo lagoon 

 from the ocean, a patch of carmeloi'te still smaller than that at 

 Abalone Point emerges from beneath the sand. Macroscopically it 

 is a light brownish or yellowish gray rock with distinct lamination 

 planes along which the rock tends to split when struck with the 

 hammer. The surfaces of this lamination plane have a greenish 

 yellow aspect, due to the abundance of a mineral of that color in 

 the vicinity of these planes. Iddingsite is the only phenocryst 

 present, and the crystals are large and well formed. 



Microscopically this facies of the rock is chiefly interesting for 

 the definiteness with which iddingsite may be recognized in the 

 ground-mass. The ground-mass is essentially glass charged with 

 minute lath-shaped crystals of plagioclase, and small, ill-defined 

 crystals of iddingsite, recognizable by their color, pronounced 

 pleochroism, characteristic cleavage, parallel extinction, etc. There 



