320 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 8, 



recesses of the hills and filled them to the same level. Other frag- 

 ments of these rocks, of an interesting character, occur in Coyote 

 Creek, in the character of natural bridges, one of which I examined. 

 The interior consists of two caverns, one on each side, connected by a 

 dark winding passage, through which the water of the creek passes. 

 The entire passage through the bridge is 240 feet. The interior of 

 the caves is lined with stalactite. 



The outside of the range bordering the valley of the San Joaquin 

 is flanked with hills of the same formation, lying in horizontal strata. 

 Looking toward the head of the valley, from an eminence at the 

 Stanislaus, I could see them extending southward as far as the eye 

 could reach, and these hills were easily distinguished by the imiform 

 level of their summits. 



Cinnabar Mines. 



Bay of San Francisco. — Being at San Francisco, I went to San 

 Jose, to visit the quicksilver-mines at the head of that valley. Find- 

 ing a steamboat at the wharf, bound for Alviso, we went on board 

 and got under way. Our course lying toward the head of the bay, 

 we had a good view of the country on each side ; and, both sides 

 having a range of hills running parallel with the bay, the scene was 

 highly picturesque. The water becomes shallow toward the head of 

 the bay ; and the low swampy country and mud-bauks of the margin 

 seem to be making rapid encroachment on the water of the bay. 

 We were landed next morning at a low muddy swamp, on which 

 the little town of Alviso is built, and we had still nine miles to 

 travel in order to reach San Jose, across a plain of rich alluvial 

 deposit. The town of San Jose is situated in the middle of a beau- 

 tiful valley, well wooded, and watered by streams from the moun- 

 tains on each side. As we passed along the valley we could not help 

 admiring the beauty of the country, the greater part of which was 

 still in its wdld state. The trees, which were principally of oak, of 

 several varieties, were thinly scattered across the valley. 



The two moimtain ridges of the Coast-range (see Map), which 

 bound the bay and the valley on each side, meet about twenty miles 

 above San Jose, and confine the view. I observed this particularly, 

 as I think it enabled me to determine the cause of the peculiar 

 character of the climate of the city of San Francisco and its imme- 

 diate neighbourhood. During the summer season, the mornings are 

 exceedingly warm mitil nearly noon, after which time the wind blows 

 in from the sea, and is felt so extremely cold, that winter-clothing 

 is found necessary. This daily variation of temperature appeared to 

 me to be satisfactorily accounted for, by supposing that when the 

 sun was near the meridian, its rays, refracted from the sloping sides 

 of the mountains, heat the air, in consequence of which it is rarefied, 

 and ascends, leaving a deficiency in the equilibrium, to supply which 

 the more dense and cold air rushes in from the sea through the only 

 opening, the Golden Gate (as the narrow entrance to the bay is 

 called) . The cold air rushes with considerable force by San Francisco, 



