ORE DEPOSIT THEORIES. 235 
Pyritic Deposits. 
It cannot be claimed that the proof of the plutonic origin 
of the class of deposits now to be considered is anything like 
so complete as in the case of tin veins, apatite veins, or 
contact metamorphic deposits. Still, in some cases there is 
a great deal of evidence bearing upon the subject. Pyritic 
deposits do not form anything like such a well-defined class 
as tin veins or apatite veins, and, of course, there is always 
the possibility when this is the case, that the various deposits 
may have originated in several different ways. As, however, 
my object is not to prove that all ore deposits have been 
produced by plutonic agencies (a task which would in any 
case be impossible), but to show that some deposits have 
been, the fact that some of these pyritic deposits may 
have originated in other ways does not affect the argument. 
Pyritic deposits are noted for their great, often enormous, 
size. Thus, the largest pyritic deposit at Rio Tinto in Spain 
measures 3300 feet in length and 600 feet maximum width ; 
the deposit at Sain Bel in France measures 4330 feet in 
length and 147 feet im width; the Mount Lyell deposit, 
Tasmania, measures 562 feet in length and 206 feet in 
width.* They are generally roughly lenticular in shape, 
but often they are almost irregular masses. Often they are, 
however, very narrow in comparison to their length. Thus, 
the famous Rammelsberg pyritic deposit is 3670 feet in length 
and 70 feet in maximum width, while the largest deposit at 
Roros, Norway, is 1170 feet in length and 10 feet in width. 
Pyritic deposits are generally parallel both in dip and 
strike to the planes of stratification of the country. Some- 
times, however, this is not the case, and very often even if 
it is, branches or tongues run out from the main deposit 
into the neighbouring strata. In the Rammelsberg deposit 
one of these branches has given rise to very heated dis- 
cussions as to whether it formed part of the original deposit, . 
or whether it had been produced afterwards by crumpling 
of the strata. Although at Rammelsberg the possibility of 
the latter having taken place has been demonstrated, in 
many other cases similar branches have been proved to form 
part of the original deposit. 
In mineralogical composition pyritic deposits vary very 
considerably. The great. bulk of the ore is usually made up 
of iron pyrites; pyrrhotite is often present in small quan- 
tities, and sometimes occurs more abundantly than any 
other mineral. Copper pyrites and other copper sulphides 
*H.J. Daly. The Mount Lyell Copper Deposits, Tas. Trans. 
Inst. of Mining and Metallurgy, 1900. 
