856 



THE GEOLOGIST. 



degree sufficient to limit the shrinking of the 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 in 

 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 infinitesimally small when compared with deposits in veins, 

 they may be regarded as instantice cnicis, indicating, at least, the direction 

 which the inquiry ought to take." Taking up the consideration of the 

 conditions connected with the Hampgill vein, where the lead ore occurs 

 in the greatest intensity, he notices that the richest portions are at the in- 

 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 between 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 elsewhere. 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 nietaUic 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 

 wbich (he 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 varving proportions 

 as a component part of the rocks of the Alston Moor district! or some still 



