ERUPTIVE BOOKS. 295 



lirui the above conclusions. In the hand specimen some of the porphyrites 

 might readily be taken for Tertiary eruptives. Among quarti- porphyries 

 the White Porphyry, which is the most acid of the group, not only has the 

 characteristics of an older rock in its internal structure, but is actually cut 

 bv transverse bodies or dikes of the Gray or Lincoln Porphyry, and a small 

 sheet of it, together with inclosing sandstones, is included in the great mass 

 of Sacramento Porphyry. An apparent exception to this evidence of the 

 earlier age of its principal mass is found in the existence of two dikes of 

 White Porphyry cutting Lincoln Porphyry, on the north wall of Cameron 

 amphitheater, but their mass is relatively very small and the occurrence 

 altogether an exceptional one. 



Among the Tertiary eruptives Nevadite seems to be older than the 

 andesites, judged by its internal structure and its geological surroundings, 

 but the rocks are so widely separated that no direct evidence was obtain- 

 able. 



Manner of occurrence. The two groups are further distinguished by the 

 fact that the older rocks are entirely intrusive and the younger extrusive ; 

 in other words, that the former never reached the surface, but were consoli- 

 dated within the sedimentary strata and under the pressure of a considerable 

 mass of overlying rocks, while the latter were, as far as can be determined 

 at the present day, actually extruded upon the surface before final consoli- 

 dation. This is an important distinction in its bearings upon the internal 

 and petrographical structure of the rocks, and one upon which it seems geolo- 

 gists have hitherto not laid sufficient stress. 



intrusive sheets. The greater mass of the older rocks occurs as sheets 

 between the strata of sedimentary rocks, generally following a given hori- 

 zon over great distances. That they were not poured out upon the surface 

 and the overlying sedimentary beds deposited upon them that is, that they 

 are not interbedded sheets is abundantly proved by the facts that they fre- 

 quently cross the strata from one bedding plane to another and that they 

 also occur as dikes cutting across the strata transversely, whose actual con- 

 nection with the intrusive sheet it was sometimes possible to observe; large 

 fragments of the overlying beds are, moreover, often found entirely included 

 in them. The great number and extent of these intrusive sheets are very 



