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PRECIOUS STONES.
1219
is common to large areas of the country around Kimberley, where rocks other than kimberlite outcrop.
It seems likely that the outcrop of rock near Oroville, designated '' kimberlite " by Mr. Cooney, is a portion of one of the belts of serpen-tinized amphibolite schists running thi'ough the country in a northwest direction, as mapped in the geological folios ° of this Survey by Turner, Lindgren, and Becker. The alluvium has been washed off, exposing a portion of such a belt, which has been mistaken for a pipe formation, since it is exposed over a limited area. The rock formation including' the region a mile north of Oroville represents a highly metamorphosed series of basic rocks which have yielded amphibolite schists and ser­pentine. In this formation are included diabase-porphyrites which would readily furnish such specimens as the "bull's-eyes" described above.
Many authenticated finds of diamonds are on record in Butte County, Cal.6 Some of these have been along Feather River not very far from Oroville. The majority have come from Cherokee Flats, north of Oroville, where Mr. Cooney and his associates own other land on which they expect to prospect. Since the presence of diamonds is well established for this part of California, it remains for some one to locate them in the matrix. Just what the nature of that matrix will be is not known. It may not be a typical kimberlite rock and in the form of a volcanic neck, but one of the other great varieties of basic igneous rocks so plentiful in the region. It has not been proved that a kimberlite formation is essential to the occurrence of diamonds, nor that where such a rock exists it must carry diamonds. Several rock out­crops are known, as in Elliott County, Ky., and Kakanui, New Zea­land, where there are basic rocks almost identical in appearance to kimberlite around which no diamonds have as yet been found. On the other hand, diamonds occur in a matrix of hornblende-diabase near Inverell, Australia. Since the composition of many of the rocks of the Oroville region is not very unlike this, it may be that the diamonds will some day be found in a matrix of similar type in that region.
Kentucky.—The revival of interest in the kimberlite rock forma­tions of Elliott County, Ky., was not abated in 1906. The Kentucky Diamond Mining and Development Company, which owns the out­crop of the kimberlite on Isom and Critches creeks, has arranged for a complete washing plant similar to those used in South Africa. This plant is at present (May, 1907) under construction, and, it is expected, will be ready to make a thorough test of the deposit within a few months. The company disclaims the knowledge of any diamonds having been found on its property. Pyrope garnet, diopside, and olivine, however, all of gem quality, have been found, along with other minerals associated with diamonds in South Africa.
Wisconsin.—The discovery of a diamond field near Plum City, Wis., has been reported.c It is said the deposit lies outside of the area of glacial drift. As yet there has been no confirmation of these news­paper reports.
"Geologic Atlas V. >.. folio 17 (Marysvilie), folio 18 (Smartsville), and folio 43 (Bidwell Bar), t". S. Geol. Survev. 6Turner, H. W., Diamonds of California: Am. Geol., vol. 23, 1899, p. 18*2. i-Jew. Circ. Weekly. Ortober 3, 190U.