86 
THE PRINCIPAL FLOOR. 
The series commences in Case 24 with a number of specimens 
illustrating the formation of veins on a small scale in nodular 
concretions of clay-ironstone, by the contraction which the 
nodules have suffered during consolidation, and the subsequent 
deposition of various minerals in these fissures of contraction. 
Not only do carbonate of lime, quartz, hatchettrine, and other 
non-metallic minerals occur in such cracks, but associated with 
these are found various metalliferous minerals identical with 
those which form the object of exploration in metallic veins ; 
such for instance, as zinc blende, galena, and copper pyrites. 
The remainder of this Case is occupied by samples of narrow 
simple veins known from their small breadth as “ strings ” or 
“ threads.” Some of these branches consist of metallic minerals, 
others of non-metallic ; and some interesting specimens are 
introduced to show the differences observable in the character 
of a vein in passing from one rock to another. 
The series of simple veins is continued in the succeeding 
Cases (Nos. 25 and 26 ), the veins, however, increasing in width 
or becoming more powerful. In many specimens introduced 
into this part of the series it will be observed that the minerals 
instead of being gathered into strings are distributed through 
the rocks themselves in an irregular manner ; this is especially 
the case with tin-stone, which frequently occurs disseminated 
through granite rocks. It has been suggested that in many 
cases the metalliferous mineral has been segregated or separated 
from the minerals of the surrounding rocks. 
From the simple veins which, with few exceptions, have 
consisted of a single mineral only, we pass in the next two Cases 
(27 and 28 ) to illustrations of lodes, consisting, first of two 
minerals, and then of several. These instructive examples are 
of high practical value, as illustrating the subject of the “ para- 
genesis ” of minerals, or the characteristic association of certain 
species, — a question of the deepest interest alike to the minera- 
logist, the geologist, and the miner. 
In some instances the formation of the different minerals has 
evidently been contemporaneous, as in the fine example from the 
Ecton mines (No. 194), in which calc- spar and copper pyrites 
have simultaneously crystallised ; but in other cases the minerals 
have obviously been formed in succession, as seen in the fine 
Brazilian veinstone (No. 203a), where we observe a sequence of 
quartz, dolomite, and magnetic pyrites. The association, how- 
ever, is in these cases, to a certain extent, irregular* and it 
remains for us to study in Case 29 those specimens in which 
may be traced a definite succession of regular deposits. This is 
strikingly illustrated by the beautiful specimens of “ riband ” or 
“ banded ” veinstone from Saxony (Nos. 232 and 233), in which 
we notice, within a very limited width, a succession of alternate 
deposits of quartz, galena, heavy-spar, iron pyrites, and zinc 
blende, the layers being repeated with tolerable symmetry on 
each side of the veinstone. 
