356 
THE GEOLOGIST. 
degree sufficient to limit the shrinking of tlie central parts of the mass. 
Into these cracks sulphides of lead, zinc, and iron, copper pyrites and cer- 
tain other minerals of a different class have not unfrequently been intro- 
duced. In such cases it seems exceedingly improbable that those sub- 
stances could be derived from exhalations from beneath. There can be, 
on the other hand, no doubt that their component parts have come by in- 
filtration from without. It is also evident, that these bodies or their com- 
ponent parts were in a soluble state when they entered the cavities formed 
by the cracks of the nodules. Infiltrations occur also in the cavities left 
by the decomposition of fossil shells, and in those formed in various other 
ways. At St. Agnes, Cornwall, crystals of feldspar have been removed 
from elvan and replaced by peroxide of tin. "Instances of this kind," 
Mr. Wallace says, " are of a deeply important character, and are almost 
conclusive that metals or the elements of metals are diffused, perhaps iu 
varying proportions, throughout the whole mass of all kinds of rocks. 
They indicate, probably, the existence of metals in veins as the result of 
combinations and changes which cannot be always, if indeed ever, directly 
connected with volcanic influences ; and although the quantities of metal 
infiltrated are iufinitesimall}^ small when compared with deposits in veins, 
they niay be regarded as insttmiice ^rwc/*, indicating, at least, the direction 
which the inquiry ought to take." Taking up the consideration of the 
conditions connected with the Eampgill vein, where the lead ore occurs 
in the greatest intensity, he notices that the richest portions are at the \n- 
tersections of the Quarter Point veins, and comes to the conclusion that 
the functions of the conditions differ from each other, those connected 
with the rich portion being favourable to a circulation of water, the other 
not so. He then proceeds to discuss the laws regulating the descent of 
water below the earth's surface, and the deposition of vein-minerals, such 
as carbonate of lime, and barytes, in the open spaces in veins. Then the 
connection is traced betAveen the laws regulating the descent and circula- 
tion of fluids and the deposition of lead-ore in the veins of Alston 
Moor. He next follows up the connection between the laws of hydrous 
agency and the deposition of lead-ore in veins on the east side of the Trent 
river, those traversing Mountain Middle Fell, and else\^here. This is 
followed by investigation of the connections of the laws of hydrous agency 
with the deposits in the " lower beds," and, considering that metallic ores 
in veins traversing clay-slate and granite, must be equally subject to the 
same agency, he devotes a chapter to them. 
Dwelling on the facts, that the richest mines are in decomposed granite, 
and that metallic ores are sometimes found in the joints of that rock, as 
is the case in the long-celebrated Carclase tin-mine, and indeed through- 
out the whole granite district of St. Austell (Cornwall), he remarks the 
correspondence as being very striking between the more decomposable 
parts of the Great Limestone in Alston ^Moor, and the decomposable gra- 
nite, in that instance being associated in the former case with pure and 
rich deposits of lead-ore, and in the latter with tin-ore of excellent quality ; 
and comments on the effects of atmospheric exercises as exhibited in all 
mineral veins generally, and in gold and silver mines more especially, sin- 
gling out the remarkable mine of Potosi as a prominent instance of the 
latter. He notices also the association of dissimilar rocks, and considers 
that in some instances the intruded rock may have been the source from 
which the metallic ore has been derived. In his concluding remarks he 
considers that from the evidence brought together it would appear that 
either lead or some basifying principle must enter in varying proportions 
as a component part of the rocks of the Alston Moor district, or some still 
