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ECONOMIC GEOLOGY 
The ore-forming solution may enter the rocks beside the 
fissures and deposit its constituents as an impregnation. 
The lode passes gradually into the country through a band 
impregnated with metallic minerals, such as the *“ capel ” 
beside the Cornish tin-lodes, the “ emborroscado beside 
the pyrite masses in Spain, and the irregular network of 
veinlets forming a stockwork. Still further impregnation re- 
places the country rock entirely by a replacement or meta- 
somatic ore. Such an ore-lode may fade. outward into the 
country, as in some Rhodesian mines, where ore rich in gold 
with no trace of the original rock constituents passes through 
ore in which the felspars can still be recognized, into country 
with only a slight impregnation of metallic minerals. One 
extreme development of replacement deposits produces the 
great pyritic lenticles (cf. Chap. VI) which may be hundreds 
of feet across, and yet include no fragment of the unaltered 
country as large as a walnut, though the structure of the 
original rock may be recognizable by the sheen on a surface 
of pyrites. 
it has been considered that ore formation is possible only 
within a shallow zone, as no spaces can exist where the 
rocks flow under the pressure. The depth of this zone is 
being steadily increased from Heim’s estimate in 1878 of 
14 miles, and Van Hise’s of 7'4 miles, to Sir Charles Parson's 
(Nature, 20th October, 1904), of at least 12 miles. His view 
was confirmed by the experiments of F. D. Adams (Fourn. 
Geol., xx, 1912, pp. 115, 117) who proved that empty cavities 
persist in granite under the pressure of IT miles deep, and 
that cavities filled with water or gas would remain at a still 
greater depth. Ore formation by the filling of cavities may 
therefore take place to the depth of at least 12 miles and by 
replacement to indefinitely greater depths. 
ORE SEQUENCE IN DEPTH—Crustified deposits often show 
a succession of different ores and veinstones in a transverse 
section of the lode. An analogous vertical succession also 
Occurs owing to zonal variation controlled by temperature. 
Some lodes have been formed at high temperatures near the 
source of the ores and plutonic water; others have been 
formed under cooler conditions near the surface. No known 
lode includes the full vertical succession of ores. At great 
depths the change in temperature and pressure is very