394 Correspondence. 



miniature icebergs, floated off by a rise in the water produced by a 

 dam of anchor-ice below. 



10. " On an altered Clay -bed and Sections in Tideswell Dale, 

 Derbyshire." By the Eev. J. M. Mello, M.A., F.G.S. 



The author describes the sequence of the rocks seen in a quarry 

 in Tideswell Dale as follows : — Beneath a thin layer of surface-soil 

 is a bed of Toadstone, containing concretionary balls, and much de- 

 composed above ; beneath this is Toadstone in large blocks of inde- 

 finite shape, very hard, dark green, and apparently doleritic, nine 

 or ten feet thick, passing downwards into a coarse and much decom- 

 posed bed, partly amygdaloid, partly vesicular, about one foot thick. 

 Beneath the Toadstone rocks, and without any sharp line of demar- 

 cation, is a thick bed of indurated red clay, three yards in thickness, 

 presenting a regularly prismatic-columnar structure, resting on a 

 thin bed of greenish-yellow clay, containing fragments of limestone, 

 which covers beds of good Derbyshire marbles containing corals. 

 The author suggests that the columnar clay -bed may perhaps be a 

 local development of that which forms partings in the limestone 

 near Litton Tunnel. 



{To he Concluded in our next Number.) 



coiai^EsiPOisriDiBisrciE. 



ME. DAVID FOEBES, F.E.S., ON EAIN AND EIVEES. 



Sir, — In the Geological Magazine for this month, p. 314, is a 

 paper, entitled " D. Forbes on Volcanos," being the substance of a 

 lecture delivered in St. George's Hall, 19th June, 1870. The 

 object of the lecture is to show that subterranean heat has, and 

 ever has had, as much to do with the formation of the surface of the 

 earth as rain and rivers have. Mr. Forbes seems to put this doc- 

 trine forward as an originality of his own, and as if it was foreign 

 to the rain and river theory. It is, however, part and parcel of that 

 theory. Mr. Forbes, p. 327, says, " British geologists of late years, 

 all but ignoring the action of internal forces, have striven to account 

 for everything in the shape of external configuration or scenery by 

 the action of water in its different forms of rain, rivers, the sea, or 

 ice ;" and again, " In answering the question, therefore, as to which 

 of these forces" (internal fire or external water) "has played the 

 most prominent part in determining the external configuration of 

 the earth, the unbiassed geologist must necessarily grant the first 

 rank to the internal volcanic or cataclysmic agencies, since had it not 

 been for their operations our globe would still have remained a 

 comparatively smooth sphere, surrounded with its external envelope 

 of water, with no visible land for the rivers to traverse or the rain 

 and ice to disintegrate and wear away ; in fact, it was only after the 

 internal agencies had produced their effects that the external forces 

 were called into play, and then became the great agents in modifying 

 the outlines of our earth to their fullest extent." 



If Mr, Forbes will only read the book entitled '•' Eain and 

 Elvers," he will find his idea stated there over and over again. 



