I'HE FORMATION OF DEPOSITS 29 
fadual; in lodes formed under such conditions the ore 
deposits may be similar through a vertical range of 6000 feet 
*S In the mines of Mysore, or of 4000 feet in the saddle- 
odes of Bendigo. The full succession in depth is determined 
7 the correlation of lodes in different mining fields. The 
heavy metals are naturally often associated with deep-seated 
Plutonic rocks such as platinum with basic and gold with 
cid varieties, The great depth at which many gold lodes 
ere formed is indicated by their vertical uniformity, 
althougp there is usually a fall in the quantity of gold but 
‘Mprovement in its quality as the lodes are followed down- 
ward, Primary tin ore is restricted to deep-seated high 
Joperature deposits. Some Cornish copper mines by 
ceper Working become tin mines, as ores of tin occur below 
those of copper. 
J Copper lodes often reach the surface, but in some cases, 
th 1 Cornwall and notably at Butte (cf. p. 91), the lodes at 
Th, Surface contained no copper but silver, lead, and zinc. 
i) three metals are usually intimately associated in ores ; 
zine © Often most abundant in the upper part of a lode, while 
"¢ increases below where the temperature was higher. 
Miva: and silver ores have a great vertical range, having 
near deposited in plutonic rocks at great depths, and also 
thro the surface, A general ascending sequence passing 
gan ugh tin, tungsten, copper, zinc, lead, iron, and man- 
i wo has been established,! but it is not universal, and is 
Dor Frees reversed. As the sequence depends upon the tem- 
for Tog of the rising solutions, variations are only natural 3 
Pauses a of igneous rocks is probably slow, with many 
A fresh uring which the overlying temperature would fall. 
tem advance of the intrusive mass would cause the high 
at Lrature minerals to invade the zone where those formed 
rp temperatures had already been deposited. The 
to thoes: vould be expected to show variations comparable 
Theo In the successive layers of a crustified lode. 
formly Pista constituents of a lode are not as a rule uni- 
enosited along it, but are collected in rich bands or 
- : ii 12; RH. 
8 E, H. Davidson, Geol. Mag. lviii, 1921, pp. 505-12; 
Rastall, Econ, Geol., xviii, 1923, p. a, H. Dewey, Proc. Geol. fn 
Hop 8925, PP. 107-35. The sequence of the veinstones is describe y 
H. B, Cronshaw, 77 LMM, xxx, 1921, p. 411.