ACADEMY OF NATURAL SCIENCES. 45 



On the Gradual Elevation of the Land in the Environs of 



San Francisco. 



BY JAMES BLAKE, M.D., F.R.C.S. 



The gradual elevation and depression of large portions of the earth's surface 

 bas, within the last few years, been attracting considerable attention from geol- 

 ogists. It is a vast geological process of which we are the actual spectators, 

 offering us the most imposing terrestrial phenomenon of which we can be cogni- 

 zant, and at the same time affording us some tangible idea of the vast periods 

 that have been required for bringing the surface of the earth to its present 

 shape. It is the general opinion of geologists that the western shore of our 

 continent is gradually rising. This has been proved to be the case as regards 

 the southern portion of the continent ; but the following facts, observed in the 

 neighborhood of this city, afford undoubted evidence that at least this portion 

 of the northern continent is being gradually elevated above the level of the 

 ocean.' 



On the northern bank of Lobos Creek, a small stream running from Moun- 

 tain Lake to the ocean, muscle shells and rolled pebbles are found at an eleva- 

 tion of from eighty to one hundred feet above the present level of the ocean, 

 and probably at the distance of half a mile from the present beach. These 

 shells and pebbles are exactly analogous to those now being deposited at 

 the mouth of the creek, and were undoubtedly placed there when the spots 

 at which they are found formed the beach of the ocean. The surface of the 

 country is so much covered by drifting sands, that it is only in spots that 

 these shore remains show themselves. The deposits first seen contain remains 

 of shells considerably weathered — lower down the creek, shells and larger peb- 

 bles are seen ; still lower down I found the same materials mixed with smaller 

 pebbles, and at an elevation of about fifty feet small bands of black peat earth 

 were found iuterstratified with the sand and gravel. These small bands of vege- 

 table earth were evidently formed near the level of the ocean by the waves 

 throwing up a barrier of sand which dammed up the waters of the creek, so as 

 to form a pond in which a layer of vegetable matter was deposited. This 

 process is going on at the present time, a dam having been thrown up by the 

 heavy storms of the winter of 1861-18G2. 



Another evidence of the recent elevation of the country is seen near the 

 western end of the Puerta Suelo, at a distance of about eight miles from the 

 city. Here there is a depression in the hills, extending from the bay to the 

 ocean, and forming a narrow neck to the peninsula on which San Francisco 

 stands. Even at present, the distance from the waters of the bay to the ocean 

 is not more than two or three miles at this point, and it is evideut that at no 

 very distant period this depression formed a channel of communication between 

 them. Near the western end .of this former channel, and at about a mile inland 

 from the present sea beach, the skeleton of the head of a whale is found on the 

 surface of the ground. The specimen measures about six feet across, and must 

 have belonged to an animal fifty or sixty feet long. The bones, which are not 

 at all mineralised, are in a good state of preservation. At the time they were 



