Pai.ache.] 



Lherzolite- Serpentine. 



171 



The second hypothesis, however, offers a satisfactory explanation of 

 all the facts observed, and is probably the correct one. The fact 

 that other masses of serpentine have been observed by the writer in 

 the vicinity, where the evidence of its intrusion into the San Fran- 

 cisco sandstone admits of no reasonable doubt, strongly corrobo- 

 rates this conclusion. The serpentine of the Potrero has been 

 described by Blake* and Newberry, f both of whom considered it 

 eruptive into the San Francisco sandstone. 



Other Occurrences of Serpentine. — -Although serpentine is a very 

 abundant rock in the Coast Ranges, there have been, so far as 

 known to the writer, but three descriptions of Coast Range localities 

 published wherein a relation has been traced between serpentine 

 and an eruptive rock. -Reference to these descriptions has already 

 been made on page 163 above. In the case of the lher/.olite-serpen- 

 tine described and figured by Mr. Wadsworth,J from Colusa County, 

 the evidence seems particularly clear and irrefutable. In view of 

 this fact it seems certain that Dr. Becker must have been unaware 

 of this description when he wrote the sweeping statements con- 

 tained in " Monograph XIII," as to the absence in the Coast Ranges 

 of serpentine of igneous origin. Mr. Turner's description^ of the 

 Mt. Diablo occurrence corresponds closely to that detailed in this 

 paper, the original rock in both localities being determined as 

 lherzolite. The specimen referred to by Mr. Merrill, || from San 

 Francisco, is doubtless from an area of serpentine geologically con- 

 tinuous with that of the Potrero, and the slight difference of min- 

 eralogical composition which places this rock with the saxonites 

 indicates merely a local variation, and not an essential difference of 

 the two rock's. 



THE INTRUSIVE ROCK. 



Occurrence. — Along the western side of the serpentine area is 

 found a belt of isolated outcrops of a dark-colored, crystalline rock, 

 extending parallel to the ridge for a mile, and nowhere more than 



* Pacific Railroad Reports. Vol. V, Pt. IT, p. 157, 1S53. 

 \Ib%d., Vol. VI, Pt. II, p. 10, 1S55. 

 % Loc. cit., p. 129. 

 \ Loc. cit. , p. 3S9. 

 || Loc. cit., p. 390. 



