300 J. LeConte— Formation of the Coast Range of California. 



I have already stated that the mountain mass lying between 

 the Bay of San Francisco and the San Joaquin plains is divided, 

 by the Amador and Livermore valleys, into two sub-ranges; 

 the Contra Costa, overlooking the Bay, and the Mt. Diablo,* 

 overlooking the plains. Both Cretaceous and Tertiary strata 

 are found in the latter, although their distribution has not yet 

 been thoroughly worked out ; but the former consists wholly 

 of Tertiary, principally Miocene. In both these sub-ranges 

 seams of lignite of good quality have been found. Those 

 found in the Cretaceous of Mt. Diablo have proved of great 

 value and are extensively worked; but as yet nothing but very 

 thin unprofitable seams have been found in the Contra Costa. 



Several months ago I was asked to examine the croppings of 

 some thin seams of lignite near the town of Hayward, which 

 had been opened to a depth of 100 to 150 feet. The coal-bearing 

 strata dip nearly perpendicularly and strike in the general 

 direction of the range. The place examined was on the 

 lowest foot hills of the Contra Costa, corresponding in position 

 to a in the section, fig. 1. 



While examining the mode of occurrence of this lignite, my 

 attention was drawn, by the intelligent Superintendent of this 

 mine, to certain slabs ^f shale in immediate contact with the 

 seam, which were litendly covered with small rounded flattened 

 masses looking somewhat like flattened pebbles. In fact he 

 supposed them to be pebbles or shingle which had fallen into 

 fissures between the perpendicular strata. Examination, how- 

 ever, quickly convinced me that they were not pebbles nor 

 extraneous matter of any kind, hut clay pellets or rioduks m the 

 original sediment which had been flattened by strong pressure in the 

 formation of the mountain range. Here then, I saw at once a 

 means of determining the amount of mashing to which the 

 sedimentary strata had been subjected in the process of moun- 

 tain-making. I immediately commenced closer examination. 



The nodules were all greatly flattened and nearly all greatly 

 elongated. Their shape therefore were mostly flattened ellip- 

 soids, though some were flattened discs. The flattened ellip- 

 soids were nearly all set on end between the strata, i. e., with 

 their long diameters vertical, though some varied considerably 

 from this position to one side or the other, and a few were 

 nearly horizontal. They were found in close contact with the 

 seam^on both sides, and some in the seam itself ; and insuch 

 numbers that they covered the surface of the strata. When 

 small and disc-shaped, or not much elongated, the surface of 

 the over-clay blackened by contact with the coal presented a 

 *The term Mt. Diablo range is usually used in a wider sense for the whole 

 range on the east side of the Bay, as distinguished from the Santa Cruz Mountains 



subdivision of this range. 



