96 GEOLOGY. 



granite gate. This was once the passage of the tidal currents between the Atlantic and the 

 Pacific. 



The protrusion of eruptive masses skirting the base of the California Cordilleras seem to have 

 been checked here by some means ; their occurrence is at least less frequent in this neighbor- 

 hood. A little further north, however, another shoot of igneous and metamorphic rocks abuts 

 against the western granite walls. They are a northwestern continuation of the Sonorian Gulf 

 Sierras, crossing the Colorado Valley in the vicinity of the mouth of the Gila, and joining the 

 California Cordilleras somewhat to the north of the before mentioned mountain pass. 



Along the eastern base of the Sierra Nevada we find volcanic activity again fully developed. 

 It not only skirts these walls of primary and metamorphic rocks, but seems to have its theatre 

 over the whole area between the Rocky Mountains and the Sierra Nevada. 



According to the accounts of American and other explorers, this vast area abounds in salt 

 lagoons, soda lakes, solfataras, geysers, warm and hot springs of various character, extinct 

 craters, and other traces of wide-spread volcanoism. 



Tertiary deposits seem not to be wanting througliDut these regions ; some of them are proved 

 beyond doubt to belong to the Miocene age. Numerous fossils, verifying such conclusions, have 

 been collected by various persons connected with government expeditions. Among other 

 discoveries, is one of the highest importance ; this is, the existence of a fossil shell (Cardita 

 planicosta) on the Pacific slope of the Sierra Nevada. This shell, originally belonging to the 

 Paris basin, is common also on the Atlantic side of this continent, and is now known to occur 

 also on the Pacific. From this we have the proof of a former immediate connexion in these 

 latitudes between the two oceans of our globe. 



Without making any further mention of other numerous geological and palfeontological facts 

 relating to those regions which have been brought to light since the last decenium, we consider 

 the area from the present head of the Gulf up to the Great Salt Lake basin as one tertiary, if 

 not quaternary sea. There were, besides the present Gulf, other inlets from the Pacific to this 

 interior sea ; some of them we know already, and others, no doubt, will be discovered a short 

 time hence. 



However wide this inland sea may have extended, we find on its western shore primary and 

 metamorphic rocks lined by tertiary strata, and on the east shore metamorphic and volcanic 

 rocks prevail. 



The bottom of this present waste, thickly overlaid with diluvial deposits, seems to have been 

 thrown out of its level by upheaving forces from below. We may call them pluto-volcanic, 

 employing a term to designate the immediate ejective- forces and the upheaving motions of a 

 general character. 



In the regions before us we have innumerable traces of ejected masses, in the shape of igneous 

 dykes and sierras of similar petrographic character, but varying in size. We find, in fact, the 

 former horizotalism of the whole Gulf basin along the Rio Grande and its tributaries, everywhere 

 traversed, intercepted and broken up by metamorphic and igneous mountain ranges. It seems 

 that the observer, in no other locality, stands in closer presence of these very pluto-volcanic 

 upheaving forces, than on the western edge of this ancient inland sea. Whoever passes over 

 this cround, particularly the desolate scenery about Carrizo Creek, if his mind should be the 

 least open to impressions of this kind, must be struck with awe! He will find himself in a 

 locality where nature gives, in a few bold words, a whole sentence of her cosmogonic history. 



