Mineralogy and Geology. oe 
The clay surface that has been mentioned, was deposited by the 
ancient lake. It is of great thickness, and now forms the substratum 
of the desert. We found numerous deep ravines extending across our 
course, deeply cut in the clay by floods from the mountains; they 
were generally narrow, but had vertical banks ofien twenty to thirty 
feet high. They could not be seen until we came within a short dis- 
tance, and this often obliged the train to make long and tedious detours. 
The ** New River” of which you speak has been well known to the 
travellers of the desert since its sudden appearance a few years ago. 
The grass that springs up along its banks has hardly an opportunity to 
attain its full growth, as it is rapidly eaten off by the large droves o 
cattle and sheep constantly pouring into California from New Mexico 
and Sonora. 
Ihave reason to believe that this is not the first instance of the 
overflow from the Colorado, and that it once flowed in a much larger 
pools known as the great and the little lagoon,” in December last ; 
there was no water in it, and that in the lakes was green and charged 
with organic matter, but made excellent coffee. 
It is probable that a local change of level produced by earthquakes 
has modified the direction of New River, and the quantity of water 
admitted to its channel. 
Sarthquakes are not uncommon in that vicinity. In Nov. 1852, two 
voleanoes burst out from the surface of the desert, and threw upa 
large column of steam and showers of mud; the country was well 
were also opened. 
I did not find the eastern borders of the ancient lake ; it is beyond 
the Colorado ; its extent and boundaries cannot be precisely determined 
until the maps of the region are completed, but itis probable that its 
area will not be less than 7,500 square miles. 
There were a series of outcrops of marine sedimentary formations 
rising ahove the general surface of the lake bed ; these were filled with 
the most extraordinary concretions of all sizes and shapes. The an- 
cient sea-drift abounds with silicified wood and marine fossils, all highly 
ished by the wearing action of sand. Numerous remarkable ex- 
