288 ANNUAL OF SCIENTIFIC DISCOVERY. 



made by loose materials, and that it is not essential to their formation that 

 the rocks and gravel, acting as chisels or gravers, should be pressed down 

 with violence or imbedded in ice, or moved forward en masse under pressure 

 by the action of glaciers or stranded icebergs. 



If it were possible it would be exceedingly interesting to ascertain the 

 length of tune required for the little grains of sand to carve the surface of the 

 granite ridge to its present form. How inappreciably small must be the 

 effect produced by a single grain 1 And yet by their combined and long-con- 

 tinued action mighty effects are produced. That the action of the grains 

 singly is not visible, is proved to us by the polished surface, for no one 

 gram cuts deeply enough to leave a scratch. Ages have doubtless elapsed 

 since this action of the sand began, and we can not tell how deep the abrasion 

 has extended : cubic yards of granite may have been cut into dust and driven 

 before the wind over the expanse of the desert. Sillimari's Journal. 



AISTCIENT LAKE IN THE COLORADO DESERT. 



Mr. "W. P. Blake, the geologist of the U. S. Pacific Eailroad Survey in 

 California, under the command of Lieutenant E. S. Williamson, U. S. Topo- 

 graphical Engineer, recently reports that a large tract of country, over one 

 hundred miles in length, at the head of the Gulf of California, has been over- 

 flowed at a comparatively recent period, and probably by the waters of the 

 Gulf.* There is evidence of the existence of a vast lake there, which occupied 

 nearly the whole area of the present desert. This lake was of fresh or brackish 

 water, as is shown by the numerous shells which are found in the thick 

 strata of blue clay forming the surface of the desert. At the base of the 

 mountains along the borders of the desert, distinct beach-lines, or water-lines, 

 were found on all the rocks, and the surface of the last was found to be 

 covered with a thick calcareous crust, nearly two feet thick in some places. 

 This calcareous coating was seen to extend up the sides of the mountain 

 spurs for a height, in some places, of over one hundred feet, and its upper 

 limit has a well-defined horizontal line, marking the former level of the water. 



Mr. Blake considers it probable that the Gulf of California once extended 

 as far north as the base of the pan of San Bernardino, 175 miles north-west 

 of its present limits, and that the deposition of silt from the Colorado river 

 has gradually accumulated opposite its mouth, so as to isolate the upper part 

 of the Gulf, and leave it in the condition of a lake, fed at tunes by the over- 

 flow of the Colorado, until at length the evaporation became more rapid than 

 the supply, and the desication of the lake was effected a result which would 

 be soon accomplished in that region of high and arid winds. This explana- 

 tion is supported by the present phenomena^ for the Colorado continues to 

 overflow, and send streams off toward the north at tunes of great freshets. 

 There is much reason for believing that a part of the desert-valley north of 

 the present lagoons, which are fed by the off-shoots from the Colorado, is 

 below the level of the sea. This subject is discussed at length in the report, 



* Preliminary Geographical Report, accompanying the Report of Lieutenant R. S. 

 Williamson, House Documents, 129, Washington, 1855. 



