no 



Prof. J. Prestwich. 



These are common geological facts. I need add bnt one more 

 instance on acconnt of the magnitude of its scale. 



Professor Clarence King, speaking of the plication of those parts 

 of the Rocky Mountains which lie in Wahsatch and Uinta, estimates 

 that the folds there measure 40,000 feet from summit to base.* What 

 must have been the contraction in horizontal distance where the 

 strata form not one but several folds, the crown of whose arches 

 attain a height to be measured by miles ! 



It is difficult to see how these corrugations of the earth's crust are 

 to be accounted for, unless we assume that the crust rests on a 

 yielding substratum, and that it is of no great thickness. For if the 

 earth were solid throughout, the tangential pressure would result not 

 in distorting or crumpling, but in crushing and breaking. ~No such 

 results are to be seen, and the strata have, down to the time of the 

 youngest mountains, yielded, as only a free surface-plate could, to the 

 deformation caused by lateral pressure. Freedom and independence 

 of motion are evident in these wonderful contortions and inversions 

 of the strata, and for that result a soft and yielding bed on which the 

 crust could move as a separate body is necessary. Nor is evidence 

 wanting that such a yielding plastic bed does exist, for rising up in 

 the central axes are not unfrequently masses of crystalline rocks, which 

 were then or shortly anterior in a viscid state, or else the strata are 

 penetrated by dykes and veins of igneous rocks indicating a still 

 greater fluidity of the broached fundamental base. 



These facts are so patent to geologists that it may seem almost 

 superfluous to adduce them. Let us suppose not entire solidity, but 

 a crust 800 miles, or even half 800 miles thick. What would be the 

 magnitude of a mountain chain resulting from the crumpling and 

 upthrow of such a mass of rock ? Where have we evidence in the 

 latest of our mountain chains of the existence of such masses ? 

 Nowhere do the disturbed and tilted strata point to a mass more than 

 a few miles thick, for the whole of the sedimentary and metamorphic 

 rocks are often uptilted, together with a portion of the molten rocks 

 on which they rested. Had the crust had the more excessive thick- 

 ness suggested by physicists and by some geologists, we should have 

 had mountains if not of greater height, at all events of greater 

 breadth ; for if a solid plate of any kind be broken and the fractured 

 edges turned up by reciprocal pressure in presence of a viscid 

 resisting material beneath, the width of the protruding mass will 

 bear a definite relation to the thickness of the plate. If, on the other 

 hand, the plate is sufficiently pliable to yield without fracture and be 

 bent into folds, the height of the arch and the width across the fold 

 will in like manner be proportionate to the thickness of the plate. 



* " Geologj of the 40th Parallel," vol. i, p. 761. 



