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of the western rim, and consisting at their bases of granitic or meta- 

 morphic rocks. The ridges appear to have been islands when the 

 desert formed part of the Gulf of California, or of the Pacific Ocean, and 

 were at the right depth beneath the surface for coral growth on their 

 summits for a long period. With the coral occurred several fossil shells 

 of forms quite unlike those of the late Tertiary of Carrizo Creek beds, 

 and apparently unlike those now inhabiting the Gulf of California. 

 They are not perfect enough for description, but may be identified with 

 some, of which the age is known, from farther east, and for that purpose 

 have been sent to the U. S. Nat. Museum, w-here eastern species are 

 preserved. They certainly do not resemble any of the fossils yet 

 described either from the Cretaceous or Tertiary beds of California, and 

 suggest that they lived in the period of which fossils are wanting west 

 of the Sierras, namely, the Eocene, or its later divisions following the 

 Cretaceous B deposits. There is an absence of anything characterizing 

 the secondary period. The shells resemble species of Cytherea, Dosinia, 

 Axinsea, Pyrula, and Tritonium of large size; also an oyster, thicker 

 and heavier than the living, or Pliocene fossil species, approaching 

 0. titan, but strongly ribbed on upper valve. It is near 0. herrmanni 

 Conrad, but seems distinct. Further collections from such coral-islands, 

 of which there may be many in the desert, will prove very important in 

 unfolding the geological history of the region, but the want of water 

 and feed for horses makes it very difficult to explore them, unless it is 

 done in the winter. Dr. E. S. Clark, who is familiar with the fossil 

 corals of the Eastern States, informs me that the species found by Mr. 

 Fairbanks cannot decide the age of their growth, being widely spread 

 in Mesozoic and Tertiary formations. 



By late letters from Dr. White it appears that the specimens men- 

 tioned above as sent for identification cannot be confidently named as 

 being any contained in the U. S. Nat. Museum, 



