84 GEOLOGY. 



larly claim our attention, and serve at the same time as the most natural division hy which to 

 unfold the entire subject. 



1st. The great preponderance of crystalline metamorphic granite pertaining to the older 



paleozoic series of rocks. 



2d. The entire absence of any member of the lower paleozoic^ or secondary rocks, in their 



regular stratified character. 



3d. The existence of extensive Tertiary deposits, forming a more or less extended flank, on 

 each side of the muntain range. 



1st. In reference to the preponderating granite formation, as exhibited in the central axis, 

 and main development of the mountain range, we shall notice a considerable diversity of form 

 and structure, but all evidently pertaining to the same general formation of metamorphic rocks 

 in their different exposures. Illustrative specimens are characterized by Professor Hall in the 

 accompanying list, to which reference may be had for special characters. 



The central axis is represented by a somewhat variable mottled granite, composed of various 

 proportions of quartz, feldspar, mica, and hornblende, frequently containing imbedded crystals 

 of tourmaline. The t xposed mass varies greatly in the degree of aggregation of its component 

 materials, assuming in some places a close sienitic texture, while in others a larger proportion 

 of feldspar renders it more readily decomposable by disintegrating causes — its exposed face 

 easily crumbling into a coarse, granitic sand. 



At other points the preponderance of mica, confusedly mixed in large scales, serves to give a 

 very irregular form to the external rock exposures. 



Belonging also to the same series, we find, particularly on the eastern side of the range, mica 

 and talcose slates associated with quartz veins. 



The irregular rocky range immediately adjoining the coast, and also probably composing the 

 numerous rocky islands extending at variable distances seaward along the same line from north- 

 west to southeast, present a distinct form of eruptive rock, described by Professor Hall as 

 '^greenstone, with soft chloritic spots, or blotches," and "porphyry, or porphyritic greenstone." 

 This character of rock forms the first extensive range of mountains east of San Diego Bay, and 

 attains an elevation in some of the higher peaks of 2,500 feet above the sea. 



Further to the north, in the vicinity of San Luis Key, several isolated peaks exhibit a 

 basaltic structure, weathering into peaked domes, with abrupt columnar faces. Professor Hall 

 considers all the rocks of this series as of quite recent origin, compared witTi the central granite 

 series above mentioned. As sustaining this view, we observe further north, in continuation of 

 this range, near Santa Barbara, evidences of disturbed Tertiary rocks associated with similar or 

 more recent igneous exposures. 



The isolated mountain peaks and ranges adjoining the Colorado River exhibit a sienitic 

 texture, which, by exposure to the dry atmosphere, acquires a deep brown, polished face, giving 

 a peculiar and forbidding aspect to the bare mountain scenery 



These sienitic rocks are frequently associated with gneiss, exhibiting a very distinct stratified 

 character, occupying a position external to the adjacent igneous rocks. 



The immediate junction of the Gila and Colorado Rivers is marked by a singular geological 

 formation. It is composed of an irregular series of rounded knolls, attaining a height of thirty 

 to eighty feet above the river level. 



