31 



On the Drifts of the Severn, Avon, Wye, and Ush. By the Itev. "W. S. 

 Stmohds, p. G. S., Rector of Pendocl, Worcestershire. 



The hardened Plutonic crust of the earth, the crystaUized skeleton of the 

 globe we inhabit, consists of rocks which the geologist beHeves were once 

 molten and in a fluid state. In the carefuUy- conducted experiments made 

 during the siaking of the Dukinfield Deep Mine, one of the deepest pits in 

 this country, it was found that a mean increase of temperature of about 

 V of Fahrenheit occurred in seventy-one feet. The influence of pressure 

 on the fusing point, and the relative conductivity of the rocks which form 

 the crust of the globe have, however, to be taken into account, and 

 the important experiments lately instituted by Mr. Hopkins, Dr. Faiebaieit, 

 and Dr. Jotjle, have led the mathematician to believe that the soUd crust 

 is at least 200 or 300 miles in thickness. 



It is upon this cooled, hardened, crystallized crust, whatever mineralogical 

 form it assumes, that the oldest sedimentary rocks are superimposed. AU 

 sedimentary strata are deposited from water, and the fact of their existing 

 proves the wear of water, and the action of the atmosphere, on preexisting 

 materials, in elaborating the sediment which settled in the hoUows of 



ancient seas. 



Deepest, and oldest kaown, is the stratified Gneiss of Sir Rodeeick 

 Mttbchisok, wHch, in the Hebrides, the island of Lewis, and the 

 north-west coast of the Highlands, is seen to underHe the equivalent 

 deposits of the Cambrian rocks of North Wales, the Longmynds, and 

 Ireland. These lower Cambrian deposits are also the precise equivalents 

 of the Laurentian system of Canada, as described by Sir W. Logan. 

 Hitherto no fossils have been found in them. 



The succeeding series of rocks indicate by their depth and thickness the 

 lapse of a long space of time between the deposition of their earUest strata 



and their last. 



The upper Cambrians furnish us with remains of sea weeds, worm 

 burrows, corallines, fossil ripple marks of ancient waves, and the impressions 

 of rain drops that pattered on the sandy shore. 



The Upper Cambrian deposits pass, by insensible gradations, into a series 

 of rocks many thousand feet in thickness. These are the Lower SUurians 

 of MuECHisoN, and afford many marine fossHs, some as highly organized 

 as Cephalopods; but no vertebrate animal, such as fish, reptHe, bird, or 

 mammal, has yet been detected in their matrix. 



