452 



changes depending upon the laws of electro-magnetism, which must 

 have been evolved by the varied actions of the elements, brought 

 into play during those movements by which the land and sea have 

 changed their places ? 



In the mean time, the Essay of M. Necker must be regarded as an 

 excellent stimulant to research 5 and judging from my own limited 

 experience, and particularly from facts observed in the mining dis- 

 tricts of the west of Shropshire during the last summer, I should infer, 

 that England will not be found deficient in phenomena amply corrobo- 

 rative of the views of Humboldt, Boue, and Necker; — Mr. Hen wood has 

 long been engaged in an inquiry, the objects of which cannot be too 

 much commended ; and you have already heard the result of a con- 

 siderable number of his most laborious investigations. It would appear 

 from these that he has already ascertained that the phenomena of the 

 mineral veins of Cornwall, do not come under those general laws to 

 which they have been referred by the native miners. As, however, 

 his labours are still in progress, it would be premature to speak of 

 the consequences to which they point, before the whole of them are 

 given to the public. 



I am here naturally led to speak of a work upon the Geology of 

 Cornwall, by Dr. Boase, composed of two parts, the former of which 

 contains most instructive and valuable detail, collected with inde- 

 fatigable industry, and is a most important addition to our previous 

 knowledge of the structure of that portion of our island. The Second 

 Part, though supported by arguments conducted with skill, and tending 

 consistently to one leading object, is directly opposed to the opinions 

 of nearly all modern geologists. Dr. Boase differs from previous ob- 

 servers, who conceived that certain granitic veins which ramify through 

 the slates have been injected into the latter 5 and supposes, since many 

 of these veins are made up of the same ingredients as the surrounding 

 slate, that the whole is of common and contemporaneous origin, the 

 veins being merely crystalline segregations. Now, without denying 

 the existence of many contemporaneous and segregated veins in 

 Cornwall as in other countries, surely no one can at this day resist 

 the accumulated mass of evidence adduced by Allan, Sedgwick, 

 Dechen, Oeynhausen, and a host of geologists, which indicates the 

 posterior intrusion of such veins as branch upwards from large 

 bodies of granite, and ramify in thin filaments through the overlying 

 killas. If, however, granitic veins be formed by segregation, and 

 if the masses of schist within a granite vein are but portions of that 

 vein under a different state of development, by what happy accident, 

 we would ask, do the angles of the entangled fragments accord with 

 each other, or with those of the wall of the vein itself? Concre- 

 tions with some approximation to regular forms, may have sepa- 

 rated themselves chemically from mineral masses to which they are 

 subordinate : but no mode of chemical action can offer us an intel- 

 ligible explanation of the angular fragments of killas imbedded in the 

 granite veins of Trewavas Head and other parts of Cornwall. They 

 can be explained rationally only on the supposition of the mechanical 



