256 Rev. 0, Fisher — On the Formation of Mountains. 



inconsiderable. I therefore conclude that the first cause suggested 

 by the author is incompetent to produce the effects attributed to it. 



It is very important to understand the reason why my conclusion 

 differs so widely from his. I will therefore repeat it. It arises 

 from the fact that Captain Hutton considers that a horizontal force 

 of compression (remembering the true meaning of the word hori- 

 zontal) would elevate a stratum of rock into an arch or dome, while 

 I consider that it could not have that effect ; but that, if the layer of 

 rock was so rigid that it could not yield to the required amount in 

 the direction of the pressure, it would be broken up into a series of 

 synclinals and anticlinals, faulted or not, and that the shortening of 

 the base would be compensated for by the heaping up of the material 

 above the original surface-level. (See Section 2, p. 252.) 



We will now consider shortly the instances adduced to give 

 verisimilitude to this theory, which requires to test it that an area of 

 given breadth, with a given thickness of deposit upon it, should rise 

 into an arch, whose height agrees with the data according to the table. 



I think that not one of these conditions is truly presented by the 

 Weald in our author's estimates. " The breadth to some point in 

 the English Channel is about 100 miles." In other words, the 

 breadth is unknown, so that we have not the datum required for the 

 first column. " The thickness of the beds is 3400 feet ; and the 

 highest part of the arch would have attained, if the upper portion 

 had not been denuded off, a height of 3600 feet above the sea." 

 There is no explanation of what beds are taken account of in the 

 3400 feet ; but since the crest of the arch, if undenuded, would have 

 attained 200 feet more than that, 200 feet of rocks not estimated must 

 lie above the sea-level in the centre of the Weal den country. Why 

 are these to be excluded ? or why rocks far below these ? Again, 

 what are we to say about the Tertiary beds which probably covered 

 some part of the Chalk, and consist of fresh- water as well as marine 

 beds, so that the whole series proves several oscillations of level 

 before the dome-shaped form was finally assumed. Are these to be 

 taken into account ? and if so, how is the depression succeeding to 

 the Woolwich beds to be allowed for ? These questions are most 

 material before any correspondence of precise measurements can be 

 pretended here between theory and fact. 



Again, is it not a mistake to regard the Wealden area as an isolated 

 dome-shaped elevation ? Is it not rather one of a series of parallel 

 corrugations, of which this one passes by Devizes, Kingsclere, and 

 Hastings, and the next south of it by Weymouth, Purbeck, and the 

 Isle of Wight, while the whole system of disturbances on this parallel 

 dates back to a period long anterior to the deposition of any of tlie 

 Wealden rocks, but probably subsequent to the Carboniferous ? While 

 the much greater distortions which affect the older rocks on that line, 

 as at Mendip and in Belgium, are the sum of disturbances previous 

 to the Wealden elevation, plus those which occurred subsequent to 

 the deposition of the Chalk. In short, we are here on an old line 

 of movement. 



With respect to the other illustration, drawn from the great 



