THE GENESIS OF ORE DEPOSITS 501 
deposits will be found in the last chapter of the new edition of 
Lindgren’s Mineral Deposits. From this it is clear that the type 
of mineralization here considered is mainly restricted in time to 
three periods, pre-Cambrian, Permo-Carboniferous, and Tertiary, 
corresponding to the greater periods of crustal disturbance and 
mountain-building. Of these the first is certainly complex and of 
great length, probably including several periods comparable in 
magnitude to the two later ones, but as yet it is hardly possible to 
disentangle these clearly in most parts of the world. The Canadian 
shield is an exception.* Here several distinct periods are dis- 
tinguishable. Nevertheless it must be remembered that although 
the now visible manifestations, including the actual deposition 
of the ores, were spasmodic, the genetic processes are undoubtedly 
in constant operation in depth, elaborating the material and pre- 
paring the way for the igneous outbreaks and accompanying ore- 
formation that occur whenever the static equilibrium of the crust 
is disturbed, whatever may be the cause to which these disturb- 
ances are due. This is a fundamental question into which we 
cannot now enter. 
tSee Miller and Knight, Trans. Roy. Soc. Canada, Vol. TX (1915), pp. 241-409. 
