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362 





R^SUM^ AND THEORETICAL DISCUSSION. 









ti/' 





minute particles, as the fragments of rock containing it are swept hither and 

 thither in the midst of the currents of water by which they are borne on- 

 ward ; but, however small these particles, they will not become oxidized, and 

 thus be lost. Platinum is another metal which is almost indestructible, and 

 one which is obtained solely from the washings of detrital material. Nowhere 

 is it known to occur in sufficient quantity in the solid rock to repay the 

 expense of its working. 



By far the most common mineral found in connection with the gold, in the 

 placer-mining operations, is the ordinary black iron sand, as it is commonly 

 called. This is apparently usually magnetite ; although, no doubt, some of 

 it comes under the head of menaccanite, which is considered to be a hema- 

 tite, or peroxide of iron, with part of the iron replaced by titanium. It seems, 

 at all events, to be a very indestructible and not easily oxidized material. 

 As magnetite is one of the most common minerals entering into the compo- 

 sition of volcanic rocks, and occurring also very frequently in granite and in 

 slates, it is very natural that it should be found in considerable quantity and 

 widely disseminated through the gold region. It is certain that it comes 

 from the debris of the lava and bed-rock, and not from the abrasion of the 

 quartzose vein-stone, for in the latter it never occurs * 



The most interesting mineral found associated with the gold in the gravel 

 of California is the diamond. This gem, as it would appear, is usually, 

 although not universally, found in gravels which are washed or searched 

 with care. Such gravels are chiefly those which are auriferous in char- 

 acter. Hence it follows that diamonds have been found in the surface 

 detritus of most gold regions, as in Siberia, Australia, along the flanks of the 

 Appalachian chain, and in California. But not one of these localities can be 

 classed among the productive ones. It appears that diamonds, like some 

 other natural products, are widely disseminated over the surface of the earth, 

 but only present in considerable quantity at a very few points. Formerly, 

 India was the productive region; then Brazil had its day; and now South 

 Africa furnishes by for the larger portion of the new diamonds put upon the 

 market. 



The mode of occurrence of the diamond is as yet but little understood, 

 in spite of the amount of research which has been expended on the subject, 

 That the diamond is " at home " in the older crystalline metamorphic rocks, 



* The writer, in examining thousands of pieces of auriferous quartz, has never found one containing either 



hematite or magnetite. 



