ON MINERAL VEINS IN CARBONIFEROUS LIMESTONE. 361 



Under the first condition, it will be very generally admitted that the 

 materials from whence those minerals were derived must have been held in 

 solution in the waters of the ocean at the different periods when the stra- 

 tified beds containing them were being deposited, and that the lime, the silica, 

 the iron, which wiU be found more or less abundantly in every formation, 

 must have been, with few if any exceptions, precipitated contemporaneously. 

 Although the ores of copper, tin, lead, and other rarer minerals which are 

 found most frequently in veins may occasionally be detected in stratified 

 beds, it is evident, from the mere traces of them that are obtained over the 

 widest areas and in geological time, that the laws which were necessary to 

 their deposition so abundantly in veius, were in their case to a considerable 

 extent inoperative. 



Physical conditions necessary to Mineral Veins. — It is only stating a 

 truism to remark that the rocks in association with which onr mineral veins 

 are found are almost entirely of marine origin ; that they once formed the 

 bottom of ancient seas, and in their undisturbed conditions were laid down 

 horizontally, and were continuous, and without the enormous rents and 

 fissures which now traverse them in every direction. The mere contraction 

 of the beds during solidification would not be sufficient to account for the 

 phenomena, as in this case the fissures would not pass down continuously to 

 any depth. It appears, therefore, clear that all veins must be due to the 

 elevation or depression of the portion of the earth's crust in which they are 

 found, and that they are younger than or formed subsequently to the con- 

 solidation of the rocks enclosing them, their age depending on the period of 

 physical change. 



It would be difficult for any one not acquainted with mining operations to 

 reahze the enormous forces that have been in operation in different ages 

 to produce the dislocations necessary for the reception of our mineral lodes. 

 To the physical geologist there can be few more remarkable phenomena 

 than mining districts present, or than the study of a weU-prepared map of 

 any of our larger districts would show. As an instance, I may mention a 

 map of the Alston district, in Cumberland, prepared by Mr. Wallace, in 

 which it may be seen that, occupying very large areas, there is a complete 

 network of fissures, the same veins being traceable through many miles of 

 country, crossed by others at varying angles. I have also shown this to be 

 the case in the Mendip district *, where the east and west veius in passing 

 parallel with one another through that Une of country are repeatedly in- 

 tersected by cross courses, and to such an extent that numerous examples, 

 especially towards Frome, may be seen in almost every limestone-quarry. 



It being admitted that through the volcanic disturbances indicated the 

 fissured character of our rocks has been produced, it will be desirable to 

 consider the influence exercised by the ocean in refiHing the veins thus 

 caused. 



On examining caverns and fissures of the Carboniferous Limestone which 

 are now open, it can at once be seen, by the honeycombed and worn cha- 

 racter of the roofs and sides, that they must at some period have formed 

 channels or passages for large bodies of water. Although in most Carboni- 

 ferous-limestone districts it is known that " swallet holes " occur, through 

 which the water now passes down and traverses certain lines of open fissures 

 below, which form the natural drainage of the country, the stream some- 

 times reappearing miles away from whence it entered, still these are excep- 



* " Abnormal Conditions of Secondary Deposits," &c., Geol. Journ. Dec. 1867. 

 1869. 2 B 



