FKOOF OF METAMOEPHISM. 131 



The stratigrapliical relations of the holocrystalHne and serpentinoid 

 rocks to nnaltered beds, in part fossihferons, at a great number of locahties 

 are such as absolutely to preclude the supposition that these masses are 

 either older sedimentary rocks or that they are intrusive. The field rela- 

 tions of the holocrystalHne rocks and of the serpentine to the sandstones, 

 as well as their microscopical character, are such as equally to preclude the 

 supposition that they represent local or regional precipitations of crystalline 

 sediments of the same age as the fossiliferous rocks. 



I feel almost justified in stating that at the Post-Neocomian epoch of 

 metamorphism the rocks of the Coast Ranges between Clear Lake and New 

 Idria contained no intrusive masses and that no eruptions accompanied this 

 upheaval. It is true that in a few cases isolated specimens of the crjstal- 

 line, metamorphic rocks simulate the microscopic appearance of eruptive 

 masses to an extraordinary degree, but these occurrences when examined on 

 the spot prove to i)ass over into manifestly metamorpliic material. Two or 

 three such pseudoeruptive rocks were discovered in the collections from tlie 

 Sulphur Bank and, after the microscopical work recorded in this chapter 

 was completed, these localities were revisited. In none of them was there 

 the slightest structural evidence of eruptivity ; in all it was manifest that 

 the suspected rock passed over into ordinary, unmistakabl}- metamorphic 

 beds within a few feet and in every direction. By no means the whole 

 country between Clear Lake and New Idria has been investigated, but so 

 many localities have been examined with care and so many reconnaissances 

 have been made into the intervening regions that it would be very strange 

 if an}^ considerable quantity of eruptive rock occupying the position indi- 

 cated had escaped detection. For, though a particular variety of lava may 

 sometimes be confined to very nari'ow limits, as is the case with the rhyolite 

 of New Almaden, eruptive phenomena, once initiated, usual!}' and perhaps 

 invariably extend over wide areas 



Epoch of metamorphism. — As will bc sliowu iu a subsequcut chapter, the age 

 of all the fossiliferous rocks associated with tlie metamorphics is Neocomian 

 Some of the most important areas of metamorphic rocks are certainly of 

 this age, and reasons will appear hereafter for supposing that no consider- 

 able portion of the metamorphic series was deposited at an earlier date. 



