Xll PREFACE. 



and Los Angeles counties; then the region of the Coast 

 ranges, from Los Angeles to Santa Barbara ; thence to San 

 Luis Obispo ; thence down the Salinas Valley and across to 

 San Juan ; thence down the Mount Diablo range to New 

 Idria, back to San Juan, and up through Alameda and 

 Contra Costa counties, on the west side of the range, to 

 Mount Diablo ; thence across the Straits of Carquines and 

 up Napa Valley as far as the Geysers, which point was 

 reached just at the setting in of the rainy season of 1861-2. 

 The party then returned to San Francisco, having been in 

 the field just a year. The chief paleeontological collections 

 made during this time were of Tertiary age. 



The explorations of the year 1861 having revealed the 

 presence of Cretaceous rocks in the Mount Diablo range, 

 and various circumstances making it apparent that the vici- 

 nity of Mount Diablo itself, by a detailed and careful study, 

 would throw much light on the structure of the Coast ranges, 

 and furnish a key to what had previously been more or less 

 obscure in the stratigraphical position of the rocks south 

 and west of the San Joaquin, we devoted a month, at the 

 commencement of the season of field-work for 1862, to 

 making a detailed examination of the region between 

 Martinez, on the Straits of Carquines, and Marsh's ranch, 

 ten miles east of the mountain. 



This accomplished, the party, in charge of Professor 

 Brewer, continued their explorations along the Mount Diablo 

 range towards the southeast as far as Pacheco's Pass, work- 

 ing up the geology and topography of an extensive district, 

 of which almost nothing had been previously known. Hav- 



