26 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Only the largest have been indicated on the map (even 

 those being necessarily made on a larger scale than that of 

 the map), where they are shown as quartzite, though also 

 including rocks from all the basement series. Those in the 

 cliff sections occur as definite inclusions, and vary in length 

 from a fraction of an inch to about 50 feet. The porphyrite 

 in other places also contains inclusions, though nowhere are 

 they so abundant as at the points mentioned. 



It is possible that some of the outcrops along the ridges 

 are remnants of a former covering to the porphyrite, but 

 undoubtedly some (if not all) of them exist as inclusions 

 within the igneous rock, as do the occurrences along the 

 shore; for some of them occur in saddles along the main 

 ridge, while those on the minor ridges are in great part 

 hundreds of feet below the average altitude of the main 

 ridge. To contain such large inclusions an intrusive of 

 considerable size must be predicated. This, together with 

 the size and form of the area of the porphyrite, points to 

 its origin as a laccolite.^ The microscopical character of 

 the rock, together with its mode of occurrence, clearly 

 indicates its intrusive nature. 



The mass of the porphyrite appears to be roughly dome- 

 shaped, with a somewhat elliptical base, and though no 

 remnant of a cover was found it cannot be doubted that one 

 formerly existed, now removed by extensive and active ero- 

 sion. The base of the mass was not seen at any point. 



Porphyrite Dikes. — Occasional dikes penetrate both the 

 porphyrite area and that of the quartzite, the latter at the west 

 end particularly. At the mouth of Silver Canon the diorite, 

 also, is cut by porphyrite dikes, one of which contains numer- 

 ous inclusions of the diorite. Wherever the directions of the 

 dikes could be determined they were found to be nearly ver- 

 tical, or within 20° of the perpendicular, and approximately 



iThe term " laccolite " is here used in the sense of a somewhat dome-shaped mass 

 which has been intruded into a yielding body of rock, not necessarily along the bedding 

 planes. On this view the undisturbed condition of the beds previous to the intrusion is 

 of minor importance, the main factors being the possession of basement and cover, and 

 the dome-shaped form of the mass. 



