Doc. No. 9.] 39 



former rocks form the principal ridge, separating the Pass of Pachco from 

 that of Santa Anna Pass, (or Canada Las Muertas), and present a very 

 rugged and conical outline at a short distance. They were probably 

 the grand centre of those recent volcanic disturbances that were in action 

 during the elevation of this part of that mountain range. The higher 

 peaks are composed of trachytic masses, while the lower hills contain 

 immense quantities of the vesicular lavas above noted ; from the position 

 which the latter occupy to the former, it appears that during the later 

 periods, when these fires were in action, fissures had been formed in the 

 sides of the two principal craters, through which has flowed large quan 

 tities of the fluid masses : one of these dikes cannot be less than eight 

 miles in length and Ian in a northerly direction. 



At the Santa Annei peaks, the principal crater has, apparently, fallen 

 in during some period of its eruption, and subsequent to this, a large 

 fissure has been fornjed, probably from the effects of an eaartnquake, 

 and, perhaps, at the time when the north side of the Grabilan was pre 

 cipitated into the de$p ravine at its base. 



North of the latter mountains at the distance of eight or nine miles, 

 is the elevated peak known as " Pacheco Peak," which is a true volcanic 

 cone, and in the rayines to the north of this crater, the Indians of this 

 section of the countiy obtain absidian for the manufacture of their ar 

 row heads ; to the north-east of the mountain a large dike of lava, similar 

 to that noticed above is found, which appears to have flowed through 

 a fissure very nearly; down to the base. 



South of the Sarita Anna, the tertiary sandstones of recent date ap 

 pear, resting directly on this class of volcanic rocks, and bear every 

 evidence of having suffered their principal disturbance from these 

 agencies ; the almost complete metamorphosis of the sedimentary group 

 is a proof of this ; and the evidences of comparative recent action is 

 found in the fact, tljat the organic remains found imbedded in these 

 rocks contain a large per centage of present existing species on the 

 coast. 



