GEOLOGICAL KEPORT. 307 



The exact nature <>t' tin* feldspathic mineral could not be determined; it does not seem 

 to be orthoclase or sanidine ; perhaps it may be albite. In their general appearance 

 they approach nearest the dioritic porphyries. They do not generally contain quartz 

 or mica, though exceptions are occasionally found, and thus, as well as by close geo- 

 graphical proximity, they merge into the other rocks of the district, especially in those 

 described last. 



Most of the rocks of adjoining districts, sometimes described as trap porphyries, 

 must probably be referred to these two groups. 



Besides these principal eruptive formations, we find numerous rocks of the pitch- 

 stone family, mostly filling veins or forming, at least, other evidently later effusions. 

 Their color is brown or black, with a resinous or semi-vitreous luster. They are gene- 

 rally brittle, and contain water as an essential component: when heated they intumesce 

 and smell fetid. Part of them contain crystallizations of feldspar, probably also 

 zeolitic minerals. 



Various other rocks were found, more subordinate and confined to only a few 

 localities, viz, basalt, phonolite, greenstone, pumice, obsidian, and others. They will 

 be described in a subjoined enumeration of the single mountain ranges. Of these 

 rocks, the basaltic, at least if they really should be such, belong to a group entirely 

 distinct from those mentioned before. 



Such rocks, which we are used to consider as the products of acting volcanoes — 

 pumice and scoria, have also been formed long before the present era. According to 

 Mr. Blake, pumice, scoria, and charcoal occur imbedded in the Miocene Tertiary strata 

 of California. Therefore their presence cannot be regarded as a conclusive evidence 

 of recent volcanic action, though such may in reality have taken place. 



The apparently complete want of distinct limits between these groups of rocks, 

 essentially differing in their extreme types, and their merging by intermediate forms, 

 by steps more gradual than are frequently found with rocks of the same group and 

 locality, has also been observed by Mr. Th. Antisell, in the Sierra Nevada and the 

 Coast Ranges (Pacific Railroad Report, vol. vii). It leads to the conclusion that the 

 subterranean agencies must have been operating during a greatly prolonged period, 

 with intervals not protracted enough to allow a material change in the condition of 

 their hearth. The mineralogical character of the rocks seems to indicate that their 

 formation began prior to the Tertiary period, and continued to the present era. This 

 inference is corroborated by evidences drawn from the relative dislocations of the 

 strata of the western continent. Single portions of the Coast Ranges of California 

 and the Sierra Nevada have undoubtedly been raised at different periods (not consid- 

 ering the first upheaval of the Sierra Nevada by the granitic eruptions). The subdi- 

 visions of the Tertiary formation hold there different relative positions at different 

 points, besides being raised, at least partly, from 2,000 to 3,000 feet above their original 

 level. The great dislocations of the strata in and east of the Rocky Mountains also 

 prove that such disturbances have taken place at various times prior, during, and after 

 the Tertiary period; and they seem to have reached their climax in the eruption of 

 these various rocks. 



By further investigation we would, probably, be enabled to draw more distinct 

 lines of separation between the different groups, and assign to them their relative age. 



