ORE DEPOSIT THEORIES. 243 
about N.W. and S.E. This porphyry has been completely 
altered to a chloritic rock, and can only be determined by 
microscopic examination; the latter reveals the presence of 
feldspar crystals and corroded phenocrysts of quartz. In 
the vicinity of the porphyry the sandstones have been 
impregnated by chlorite, and converted into chlorite schist. 
This chlorite schist and the chloritised quartz-porphyry are 
practically indistinguishable to the naked eye. 
Although no axinite, fluorite, or other minerals containing 
boron or fluorine have yet been found in the Barn Bluff 
deposits, I think that their resemblance to the Colebrook 
deposits in other respects is so striking that it is impossible 
to suppose that they can have had an essentially different 
origin. 
_ I regret that I am unable to discuss the origin of the 
many other large pyritic bodies on the West Coast of Tas- 
mania, such as the Mt. Read and Hercules, the Rosebery, 
the Mt. Black, and the world-renowned Mt. Lyell and North 
Lyell, the numerous deposits at Mts. Darwin and Jukes, 
Tyndall, and the Red Hills. Although I have visited most 
of these localities, I have not yet had the opportunity of 
studying them exhaustively, and without very exhaustive 
examination it is impossible to arrive at any conclusion 
with regard to their origin. 
e 
True Fissure Veins. 
It will be impossible for me to give a complete review of 
the evidence which has been deduced for the plutonic origin 
of fissure veins. This is the most complex, and probably the 
most important, class of ore deposits with which we have to 
deal, and it is also that class upon which the greatest differ- 
ences of opinion exist among geologists as to the question of 
origin. I propose here to describe only a few striking 
instances in which veins containing the metals, copper, lead, 
zinc, silver, gold, have been proved to be of plutonic origin. 
We will then be in a position to briefly discuss the question 
from a theoretical point of view. 
Copper Pyrites Veins. 
There are some kinds of copper pyrites veins which are so 
directly connected with tin veins, that it is not possible to 
imagine that they can have been deposited by any other 
solutions than those which deposited the tin. The most 
instructive case is that of the tin-copper veins of Cornwall, 
in Great Britain. In the upper levels many of these veins 
were worked for copper alone, tin being practically absent. 
In depth, however, the tin content increased, while the 
copper content fell away, so that during the process of 
