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238 PROCEEDINGS OF SECTION C. 
eruption of the granite by gaseous or watery solutions acting 
under extraordinarily high pressures, we may well imagine 
that where fissures are absent the solutions would be forced 
along the planes of stratification of the country. Further, 
such strata as were especially porous, or especially favourable 
to the processes of metasomatic replacement, would be pro- 
ferentially selected by the upcoming solutions. No doubt 
any fissures which were in existence would be fully used ; 
and many of the deposits have certainly been formed along 
fissure planes. In other cases, although the solutions may 
have come up along fissures, deposition has qnly taken place 
along certain beds favourable to metasomatic action, which 
are intersected by the fissures. Under the very high pres- 
sures which would be present during the formation of these 
deposits, it is probable that open fissures could not exist. 
Under the circumstances, the planes of fissuring might not 
afford a much more favourable channel of escape for the 
solutions than specially porous beds in the sedimentary 
rocks. It seems also probable that, during the process of 
mountain-building, the fissuring itself would take place 
largely along the planes of stratification. 
The general approximate conformability of the pyritic 
deposits with the surrounding sedimentary rocks is therefore 
not an argument against the plutonic origin. though I 
cannot regard it in the present state of ovr knowledge as a 
strong one in its favour. 
Notwithstanding these arguments, there are some geolo- 
gists who still adhere to the sedimentary origin of certain 
deposits; such, for example, as the pyritic deposits of 
Rammelsberg, where the non-conformability has not been 
proved. I do not propose to discuss this question in the 
present paper. As far as my present argument goes, it is 
sufficient to know that many deposi‘s have certainly not 
had a sedimentary origin. 
An extremely interesting group of pyritic deposits occurs 
at North Dundas, on the West Coast of Tasmania. The 
West Coast is now well known on account of its large 
deposits of sulphide and pyritic ores; but it is not so well 
known that some of these deposits contain very considerable 
quantities of tin. The principal importance of these pyritic 
tin-copper deposits from a genetic point of view lies in the 
fact that they appear to form a connecting link between 
deposits which are of certainly plutonic origin and deposits 
for which a plutonic origin cannot be directly proved. 
At North Dundas the source of the tin is undoubtedly 
to be referred to numerous dykes of tourmaline quartz- 
porphyry, which penetrate both the older sedimentary slates _ 
