78 Cretaceous. 



Near these cai'bonaceous strata, and above them, is a narrow belt, 

 partly altered and folded, and from 150 to 200 feet in width. The 

 Rodes Valley marks the limit of the Cretaceous going west from Mar- 

 tinez, the Tertialy succeeding in that direction, and resting conform- 

 ably on the strata beneath, and having the same general southwestei'u 

 dip. South of Martinez, the Cretaceous strata have a higher dip, but 

 in the same direction. 



Near the " Walnut Creek House," a small patch of Cretaceous 

 occurs, extending over a few acres, from which the overlying Tertiary, 

 forming the crown of a low anticlinal, has been denuded. 



Monte Diablo is one of the most conspicuous and best known land- 

 marks of California. The central mass is made up of metaraorphic 

 rocks; it is about six miles long, and 1^ miles in widtli, and is sur- 

 rounded on all sides by entirely unraetamdrphosed strata. It is of an 

 irregular crescent form, the concave side turned to the north northeast. 

 It consists essentially of a central portion of very hard metamorphic 

 sandstone, containing considerable epidote, flanked on both sides by 

 jaspers, silicified shales and slates. The former constitutes the north 

 peak, the latter the main peak, on Monte Diablo itself. Along the 

 flanks of the ridge of which Eagle point is the culmination, one may 

 observe the gradual passage of the argillaceous sandstone into the 

 hard dioritic or trappean rock. The strata may be traced in all stages 

 of passage, from the soft sandstone to the hardest and most crystalline 

 rock. On the outside of the great central metamorphic mass, both on 

 the north and south, are heavy accumulations of jaspery rock, one of 

 the most peculiar features of the mountain, and the material of which 

 the culminating point itself is made up. The jasper varies in color 

 from a dull brick red to a brilliant vermillion hue, and m.ay be traced 

 in the ravines in which Bagley creek heads, passing into the unaltered 

 shales of undoubted Cretaceous age, containing Ammonites Inocera- 

 mus, and other fossils. These jaspers are evidently the result of the 

 alteration of the Cretaceous shales. Gold, copper and cinnabar have 

 been found in these metamorphosed rocks. 



The unaltered Cretaceous strata, consisting of shales, sandstones 

 and argillaceous limestone, flank the sides of Monte Diablo, and run 

 'out into the plains of the San Joaquin in long, low, and almost paral- 

 lel ridges. Coal has been found in the shales, but the extensive work- 

 able beds ax'e included in the sandstones belonging to the upuer part 

 of the Cretaceous. The Arroyo del Puerto, Lone Tree Canon, and 

 Hospital Canon, cut through sandstones and shales of Cretaceous age. 

 The summit of Mount O^ is composed of jaspers, generally dull red, 



