236 PROCEEDINGS OF SECTION C. 
are always present in small quantities. Galena and zinc- 
blende are sometimes present in considerable quantities, 
but generally only in traces. Of the non-metallic minerals 
which occur in pyritic deposits or associated with them, 
the following are the most important :—Quariz, barite, car- 
bonate spars, magnesia and potash mica, actinolite, chlorite, 
talc, tourmaline, fluorspar, chondrodite, and axinite. These 
vary very greatly in the amount in which they occur in 
various deposits, and none of them can be said to occur in 
all, except, perhaps, quartz and chlorite. Generally the 
metallic minerals exceed in amount the non-metallic 
minerals very greatly. 
The structure of the deposits is frequently banded. This 
used to be alwaysconsidered to point toasedimentary origin ; 
‘now it is more generally believed to be due to the fact that 
sedimentary rocks have undergone replacement under the 
influence of mineralising solutions. In many deposits the 
pyrites is quite dense, as, for example, in many of the deposits 
in Spain and the Mt. Lyell deposit in Tasmania. In these 
cases it is uncertain whether the deposits have been formed 
by metasomatic replacement or have been deposited in open 
cavities. 
Pyritic deposits are almost always connected with eruptive 
rocks, and sometimes there is proof that they have been 
formed before the final stages of consolidation, for they are 
sometimes intersected by the dyke-forming representatives 
of the eruptive rock with which they are connected. They, 
however, do not show any preference for any particular 
rock. Thus, the Norwegian deposits are always found in 
connection with sausurite gabbro; the Rammelsberg deposit 
probably with granite; the Spanish with either basic or 
acid rocks. In this case both acid and basic rocks occur 
in the vicinity, and it is not known to which class they are 
. to be referred. In the Mt. Lyell district the deposits are 
associated with acid rocks, quartz porphyries, or felsites, and 
also with rocks of intermediate basicity, syenite porphyries. 
It is remarkable that in three districts in which these pyritic 
deposits occur, namely, Norway, Spain, and Tasmania, the 
eruptive rocks (in all cases probably intrusive, and in one 
case, Norway, certainly so) are frequently interbedded with 
the sedimentary rocks or schists. 
The argument in favour of the plutonic origin of these 
deposits may be briefly stated as follows :— 
1. Almost all of these deposits occur in the vicinity of 
eruptive rocks. The fact that they are not specially bound 
to one kind of rock, like the tin veins or the apatite veins, 
is in accordance with what we should expect. The principal 
