460 



THE GEOLOGIST. 



of primitive granite. ... A considerable port ion of our profitable niiiiiiig'- 

 operations is eai-ried on eitlier in, or contiguous to, this secondary f!:ranite 

 (never extending into what I denominate i^rimitive). As the produclive 

 granite commonlv occupies the base of lofty hills, and the margin of some ex- 

 tensive granite districts, so the bold prominences and unbroken central portions 

 may be safely assumed to be essentially of a primitive character." 



" No firt mine, yielding a profit, has hitherto been found exce]')t in secondary 

 granite, or in very quartzose or micaceous clay-slate, comiected or unconnected 

 vrith elvan. . ^ . Coj^per ores are much more ext ensively diffused, and good 

 mines of this metal have been found in secondary granite, compact clay-slate of 

 various colours when granular and containing a large portion of felspar, and in 

 greenstone. Lodes in dark coloured killas are most productive aaIicu above, 

 passing thi'ongh, or a little below elvan-com'ses. At much depth below 

 the etvan they ai'e seldom rich, unless another elvan-coui-se, or granite be 

 situated below it still. 



"After many years' experience, and careful observations, made in all the 

 mining-districts of Cornwall and Devon, I have come to the conclusion that the 

 two kinds of granite which I have designated as prunitive and secondary, differ 

 as much, in many respects, as granite and elvan ; that primitive granite con- 

 tains no metallic ores of value ; that tin ores are found nearest to it ; and cop- 

 per ores of value never in it, nor veiy near to it." 



Merely taking exception to the words immitive and secondary, we othervdse 

 fully recognize the truth and importance of Captain Thomas's classification, 

 which is a'highly important one, and most creditable to his powers of observa- 

 tion. The " secondary" granite is that decomposed and altered portion con- 

 stituting the " contact ed^es" and " upper surfaces" of the main, compact, or 

 "primitive" mass, which latter according to Prof. Cotta's hypothesis would be 

 deprived of all metallic contents by its more slow cooling; while the former, or 

 " secondary" portion, which may vary in width according to circumstances, is, 

 with the sedimentary rocks in its neighbourhood, and their associated " por- 

 phyries," or " elvans," exactly where, according to the same hypothesis, we 

 should expect to find the metals most abundant, and where, according to the 

 testimony of Capt. Thomas, they are in fact found in the rich mines of Cornwall. 



As oui- limits are exhausted, we shall only refer to one point more. Captain 

 Thomas strongly objects to the hypothesis that metals are probably derived from 

 beneath. The reason he gives for this objection is that the deepest granite is 

 the most unproductive of metalliferous ores. " The Cornish and Devon mines 

 of all kinds," he says, " are found in strata of different sorts, includhig patches 

 of a certain kind of granite, lying upon the everlasting rock, the primitive, the 

 unmoved granite — never in it." Leaving out of the question the mistaken 

 notion which Captain Thomas seems still to hold of all granite being " pruni- 

 tive," whereas the Cornish gi-anite is comparatively recent, the objection is not 

 an unnatiu-al one. But it is completely met by Prof. Cotta's hypothesis, as 

 pointed out by ]\£r. Salmon in his article in our present number, to wMch we 

 refer our readers. 



_We have spoken freely on the important subject of the complete want of 

 scientific education among Cornish miners. It is a lamentable thing to see so 

 much natural good sense, such great practical experience, and such unparal- 

 leled opportunities of observation, lying comparatively barren and unproductive 

 to the progi-ess of science ; or, stni worse, being often absolutely a bar to its 

 advance, by lending itself to contemptible charlatanism. 



