780 CONCLUDING EEMARKS. [Oh. XXXVIII. 



other metals are found in dikes of basalt and greenstone, as well as in 

 mineral veins connected with trap rocks, whereas tin is met with in 

 granite and in veins associated with the granitic series. If this rule 

 hold true generally, the geological position of tin in localities accessi- 

 ble to the miners will belong, for the most part, to rocks older than 

 those bearing lead. The tin veins will be of higher relative antiquity 

 for the same reason that the "underlying" igneous formations or 

 granites which are visible to man are older, on the whole, than the 

 overlying or trappean formations. 



If different sets of fissures, originating simultaneously at different 

 levels in the earth's crust, and communicating, some of them with 

 volcanic, others with heated plutonic masses, be filled with different 

 metals, it will follow that those formed farthest from the surface will 

 usually require the longest time before they can be exposed super- 

 ficially. In order to bring them into view, or within reach of the 

 miner, a greater amount of upheaval and denudation must take place 

 in proportion as they have lain deeper when first moved. A con- 

 siderable series of geological revolutions must intervene before any 

 part of the fissure, which has been for ages in the proximity of the 

 plutonic rocks, so as to receive the gases discharged from it when it 

 was cooling, can emerge into the atmosphere. But I need not enlarge 

 on this subject, as the reader will remember what was said in the 

 30th, 34th, and 37th chapters, on the chronology of the volcanic and 

 hypogene formations. 



Concluding Remarks. — The theory of the origin of the hypogene 

 rocks, at a variety of successive periods, as expounded in two of the 

 chapters just cited, and still more the doctrine that such rocks may 

 be now in the daily course of formation, has made and still makes its 

 way, but slowly, into favor. The disinclination to embrace it has 

 arisen partly from an inherent obscurity in the very nature of the 

 evidence of plutonic action when developed on a great scale, at par- 

 ticular periods. It has also sprung, in some degree, from extrinsic 

 considerations ; many geologists having been unwilling to believe the 

 doctrine of transformation of fossiliferous into crystalline rocks, be- 

 cause they were desirous of finding proofs of a beginning, and of 

 tracing back the history of our terraqueous system to times anterior 

 to the creation of organic beings. But if these expectations have 

 been disappointed, if we have found it impossible to assign a limit to 

 that time throughout which it has pleased an Omnipotent and Eternal 

 Being to manifest his creative power, we have at least succeeded be- 

 yond all hope in carrying back our researches to times antecedent to 

 the existence of man. We can prove that man had a beginning, and 

 that all the species now contemporary with man, and many others 

 which preceded, had also a beginning. 



It can be shown that the earth's surface has been remodelled again 

 and again ; mountain chains have been raised or sunk ; valleys formed, 

 filled up, and then reexcavated ; sea and land have changed places ; 



