Frequently there have been rumors of the discovery of tin stone in San
Diego County, but up to this time no such ores are known to exist
outside of the property known as the Temescal Mines, which are located
on the line of San Bernardino and San Diego Counties. However, in the
large crystalline area which extends over a considerable portion of San
Diego County, it would not be strange to find tin stone under conditions
similar to those which obtain in the Harney Peak region in the Black
Hills of Dakota. There the cassiterite is found in good-sized crystals
and finely disseminated grains in a matrix of coarse-grained granite,
which has been thrust up through the schists in the form of intrusive
dikes of greater or less size. It usually is associated with tourmaline,
wolfram, and other black minerals, making it difficult to distinguish it
without making a test of some sort. The search for tin in San Diego
County has been confined to looking for a black rock similar to that
found in the Cajalco Mine.