RESUME. 171 
From Cactus Pass to the junction of Bill Williams’ fork with the Rio Colorado, we cross suc- 
cessively three er four chains of mountains running from north to south, and crossing the chains 
of the Mogoyon system. ‘These mountains, which belong to the system of the Sierra Nevada, 
and which we called the Cerbat mountains, are formed entirely of eruptive and metamorphic 
rocks, with some beds of conglomerate and red clay belonging to the Tertiary epoch. I have 
recognised along the course of Bill Williams’ fork several veins of argentiferous lead—an indi- 
cation that silver is common in these mountains. 
From the Rio Colorado to Menté we cross a country of mountains, formed almost wholly of 
granitic rocks, with the exception of three plateaux, which are occupied by sandstones or lime- 
Stones, and modern sand. From the point where we quit Mojave creek to Cajon Pass, there is 
a plateau formed by a white conglomerate sandstone of diffuse stratification, and much upheaved 
by the Sierra Nevada. This sandstone is evidently Tertiary and posterior to the Eocene. 
From Monté to Los Angeles, and at San Pedro, the road is constantly over modern alluvium, 
which probably conceals beds of the Tertiary epoch. 
In the Cajon Pass I found sienite, trap, and serpentine, exactly similar to those found between 
Rough-and-Ready, Grass Valley, and Nevada City, and which contain the veins of auriferous 
quartz. 
As specimens were given to me at Los Angeles, very rich in gold, coming from the Cajon 
Pass, it is more than probable that this point will one day be one of the richest places in 
California. 
In an economical point of view, the eruptive rocks which form almost the whole country 
between Cactus Pass and Cajon Pass will furnish excellent materials for construction—for 
bridges, roads, and houses ; there are also very beautiful marbles, red porphyry, and especially, 
I think, will be found there mines rich in silver and gold. 
Before concluding, I will say that the relative age of the Sierra Nevada is much less than 
that of the Rocky mountains, although the direction of the two chains is the same—that of the 
meridian. The Coast range was raised at the end of the Eocene epoch, whose beds it has 
upheave: land disloeated, as may be seen in the environs of Monterey ; and the Sierra Nevada 
was raised later—at the end of the Miocene, or Pliocene. I have not been able to determine to 
which of these two this system of dislocation it corresponds. 
Accompanying this will be found a geological section of the country passed through, as cor- 
rect as consistent with the short time I have left in making it. 
I am, dear sir, your most obedient servant, 
JULES MARCOU, 
Geologist and Mining Engineer of the Southern Pacific Railroad Survey. 
A. W. WHIPPLE, 
First Ineut. Top. Engineers, U. S. A 
in charge of Kamlocoion of Houte near 35th Parallel. 
