66 



University of California. 



[Vol. i. 



mass with occasional patches of granular material, and numerous 

 phenocrysts of feldspar and grains of magnetite. 



A section of this phase is illustrated in Fig. 5, Plate 5. 



In places this porphyritic glassy rock is wholly or largely made 

 up of curious spherical masses varying in size from an eighth of an 

 inch up to three inches in diameter. On a weathered surface these 

 spheroids stand out prominently and they may frequently be wholly 

 separated from the surrounding rock. Their surface is rarely 

 smooth, but in the larger ones always more or less knobby or reni- 

 form. In some cases two will coalesce, producing a mass shaped 

 like a dumb-bell. 



The smaller ones are solid and in section present a radial struc- 

 ture more or less perfect. Those of larger size are nearly all hol- 

 low, having a crust of the rock substance and within, a lining of 

 chalcedony, which sometimes fills the whole space or incloses a cen- 

 ter of quartz, calcite, or marcasite. A thin section of the rim of 

 one of these hollow spheroids shows it to consist of the same 

 materials as the main mass of the rock, but in a more advanced state 

 of crystallization. The phenocrysts are more abundant and the 

 ground-mass consists of long radial aggregates of fibers grouped 

 about points on the interior surface of the shell, with a small 

 amount of interstitial glass. These fibrous aggregates do not afford 

 a distinct black cross between crossed nichols, but exhibit a very 

 feeble undulatory extinction. 



The spheroidal masses are probably of the nature of spherulites, 

 though having- some characters not common to such structures. 



o fc> 



Distribution of the Different Facies. — The three facies of the rock- 

 described in the foregoing pages are very unequal in their relative 

 abundance. 



The spherulitic type is the dominant form of the rock, compos- 

 ing not less than two-thirds of the bulk of the sheet. The southern 

 portion of the formation is almost wholly made up of this type, and 

 patches of it are found over the whole area of the sheet. 



In the northern portion of the area over which the formation is 

 found the porphyritic type prevails, mingled with more or less of 

 the spherulitic facies. 



The glassy facies is confined to one small outcrop in the southern 



