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THE GEOLOGIST. 
treme case of this breccia-vcin ; and also the peculiar manner in which 
these fragments are usually surrounded by some crystallized mineral, 
marked by the axes of the crystals all radiating as if from a centre. 
The next figure shows the banded or layer-like form, which is 
characterized by the mineral-layers being regularly separated from 
each other, but without any break, and symmetrically disposed in a 
stratified form, so that the same layers repeat themselves on both 
sides from the walls to the middle. 
It is characteristic of these two forms that when drusy cavities, or 
" vughs," occur in the former, they are distributed irregularly 
through the vein ; while in the banded form they are always con- 
tained in the middle. These distinctions are shown in the figures. 
IX. Cotta's Hypothesis.- — If we now examine Prof. Cotta's hypo- 
thesis we find that he holds : 
1. — That the metals originate from heloiv, out of the eruptive rocks 
which, accepting the theory of gradual refrigeration, are assumed 
to form portions of the original fluid nucleus. In objection to this 
hypothesis it has hitherto been urged that such an assumption was 
inconsistent with the observed facts, inasmuch as although the 
