activity that are commensurate with the production of the features manifested, but 

 it should be remembered that the area thus affected is not applicable to the en 

 tire suite of fossiliferous rocks incident to these and the ranges of the coast 

 mountains. 



We come now to consider another group of sedimentary rocks, skirting the 

 base and foot-hills of these mountains and confined to the county of Los Angeles 

 and northern part of San Bernardino. These were traced from the mountains 

 lying between Arroya Peros and Rio Santa Clara on the west to a point east of 

 Los Angeles as far as the Arnaci and San Jose ranches lying to the east and 

 south of the Monte. 



To convey a clear idea of the position of these rocks, we will commence to 

 the north of the city of Los Angeles, and at the base of the higher hills, or 

 where the sandstones and slates are found in direct contact with the primitive 

 and other igneous rocks. 



As before observed the principal rocks of this chain are granitic. The first 

 rocks of sedimentary origin met with and in contact with the granite is a bed 

 of arenaceous slates. These so far as I was able to form an opinion appeared 

 to be about thirty-five feet in thickness and dipping south fiity-five degrees. 

 Beyond this, coarse sandstones were met with having the same dip interlamina- 

 ted with their beds of fine brownish clays. These rocks apparently have been 

 but little changed by subsequent volcanic intrusions and contained many fossils, 

 all of which were marine consisting mostly of the cardium and allied species. 

 Advancing south we now approach the first foot-hills of the valley and such as 

 are found within one mile of the town ; here a change of character specifically 

 different from any of the aqueous rocks to the east, is to be observed. This lat 

 ter suite constitutes the only material change of consequence, among the tertia- 

 ries of the San Bernardino chain. 



By way of distinction these rocks will be denominated the infusorial group, to 

 separate them from the earlier and later rocks of this era, and as there will be 

 occasion to speak of them as occupying the position of distinct groups, belong 

 ing to one or more of the periods. These rocks are made up of beds of sand 

 stone 100 feet, having a buffy yellow color and which pass almost imperceptibly 

 into a sandy clay-slate, and thence into a fine, whitish, soft and light chalk-like 

 deposit 16 teet, perfectly stratified and often receiving the name of chalk day ; 

 this is again covered by a light fossil slate 28 feet, containing fragments of small 

 and as yet undetermined species of animals ; the dip of these rocks is very uni 

 form, maintaining nearly a vertical position or but slightly departing from it. 

 On these rests the drift 30 feet, which in hills fronting the plains forms an imper 

 fect coarse conglomerate, made up of the sienites, granites, trap, indurated and 

 metamorphic sandstones. All this suite have been disturbed by recent intru 

 sions of trap rocks and as in the case of the buffy sandstone half a mile north 

 of the city, the transition near the points of contact is such that the rock will 

 fracture transversely to the lines of stratification as readily as in any other di 

 rection, the fracture is always conchoidal, breaking with sharp well-defined edges, 

 and the stones when broken having a sharp sound much resembling the dolorites. 

 The high inclinations of these beds are due undoubtedly to the later intrusions of 

 the igneous rocks, and they must have received their dip anterior to the deposit 

 of the coarse drift, as the latter appears to rest unconformably upon their up 

 turned edges. This is the fact with respect to this entire group generally, 

 whether found in the county under consideration, or in any other part of the 

 State, in which it has been observed. 



The artesian boring in the city of Los Angeles has developed to a certain ex 

 tent the character of the plain beneath the surface to a depth of 400 feet ; the 

 position of the strata through which they have descended is as follows (as near 



