4G0 
THE OEOLOOIST. 
of primitive granite. ... A considerable portion of our profitable miuing- 
opcrations is carried on either in, or contiguous to, this secondary granite 
(never extending into what I denominate primitive). As the productive 
granite commonly occupies the base of lofty hills, and tlie margin of some ex- 
tensive granite districts, so the bold prominences and unbroken central portions 
may be safely assunu>d to be essentially of a primitive character." 
" No tin mine, yielding a protit , has hitherto been found except in secondary 
granite, or in very quartzose or micaceous clay-slate, connected or unconnected 
with cl van. . . . CVy;/;(";- ores are much more extensively diffused, and good 
mines of this metal have been found in secondary granite, compact clay-slate of 
various colours v\ hcn granular and containing a large portion of felspar, and in 
greenstone. Lodes in dark coloured kiUas arc most productive when above, 
passing tlirougli, or a little below elvan-eourses. At much dej)th below 
the elvan they are seldom ricli, unless another elvan-course, or granite be 
situated below it still. 
" After many years' ex])erieuce, and careful observations, made in all tlie 
mining-districts of Cornwall and Devon, I have come to the conclusion that the 
two kinds of granite which I have designated as pruuitive 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 arc foxmd nearest to it ; and cop- 
per ores of value never in it, nor very near to it." 
Merely taking exception to the words primiike and secondari/, we otherwise 
fully recognize the truth and im]iortauce of Cajjtaui Thomas's classification, 
which is a higlily important one, and most creditalDle to his powers of observa- 
tion. The " secondary" granite is that decomposed and altered portion con- 
stituting the " contact edges" and " upper siu-faces" of the main, compact, or 
"primitive" mass, which latter accorduig to Prof. Cotta's hypothesis would be 
deprived of aU metallic contents by its more slow cooling ; wliile the former, or 
" secondary" portion, which may vary in width according to circumstances, is, 
with the sedimentary rocks in its neighboui-hood, and then- associated " por- 
j)liyries," or " clvaus," exactly where, according to the same hy]oothesis, 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 Comwall. 
As oiu- limits are exhausted, we shall only refer to one point more. Captain 
Thomas strongly ol)jeets 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, " arc found in strata of different sorts, including patches 
of a certain khid 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 liold of all granite being "pruni- 
tive," whereas the Cornish granite is comparatively recent, the objection is not 
an umiatural ouc. But it is completely met by Prof. Cotta's h'ypotliesis, as 
pointed out by Mr. Salmon iu his article in our present number, to wliich we 
refer our readers. 
We have spoken freely on the important subject of the complete want of 
scientific echication among Cornish miners. It is a lamentable thing to see so 
much natural good sense, such great practical experience, and such imparal- 
leled opport unities of observation, lying comparatively barren and unproductive 
to the progress of science ; or, stili worse, being often absolutely a bar to its 
advance, by lending itself to contemptible charlatanism. 
