294 CALIFORNIA ACADEMY OF SCIENCES. 



or black. They are usually very thin-bedded, averaging 

 about an inch, but ranging from a fraction of an inch to 

 several inches. They are very hard, and break up into 

 little parallelopipedons. Almo5t everywhere they are ex- 

 posed, the strata are very much and very characteristic- 

 ally contorted, yet maintaining their bedding almost 

 unbroken, like the edges of a bale of cloth which has 

 been crumpled up. 



It has recently been shown that in places the phthanite 

 contains an abundance of Radiolaria, though in poor pres- 

 ervation.* 



From this it has been thought that the phthanites were 

 originally silicious deposits, the silica having been derived 

 from organic remains. 



Occurrence. — Small outcrops of phthanite are abundant, 

 and it frequently appears to be all mixed up with the 

 sandstone, serpentine, etc., as though in the crushing to 

 which the metamorphics had been subjected the phthanite 

 bed had been torn into small masses which had been 

 thoroughly scattered among the other beds. That we do 

 not observe the same thing for the other beds is doubtless 

 due to the fact that, as a rule, we have no way of noting 

 the lack of relation between adjacent outcrops or masses. 

 These small outcrops are scattered all along the foothills 

 from Milbrae to Haakerville, more especially on the 

 edges of the large outcrops of serpentine. The low hill 

 at Point Coyote and all the hills close to Belmont and San 

 Carlos are phthanite, and on most of them may be found 

 good exposures. There are besides a few scattered ex- 

 posures — on the ridge between Calera and San Pedro 

 Valleys close to the road, on the San Francisquito Creek 

 near Searsville, etc. There are many good exposures of 

 phthanite about San Francisco. It is the predominating 



* Bull. Dept. of Geol., Univ. of Cal., vol. i, pp. 199, 200. 



