280 Mr. P. J. Martin on the Anticlinal Line of 



outcrop of all the strata concerned ; and particularly of those 

 atony strata in which we should be most likely to find the con- 

 joint signs of disruptive violence and of aqueous erosion, distin- 

 guishable from the detrital operations of time and weathering. 

 Passing by the well-known fact, that where denudation has been 

 active, the prominence, or the want of it, of any given stratum or 

 order of strata is in exact proportion to their induration, or their 

 resisting power, — if soft and destructible, the surface being re- 

 ceding and low, if hard and stony, hilly and high,— we fix our 

 attention first on the chalk. 



The soft and destructable nature of the material, whilst it pro- 

 duced the smooth outline of the chalk hills, has so determined 

 the fonn and constitution of their escarpments, that they exhibit 

 no signs of laceration beyond their coved and scooped surfaces. 

 The sharp angles and fracture edges which convulsion had left, 

 atmospheric agencies have obliterated. The homogeneous struc- 

 ture of the rock-masses of the chalk has also determined the 

 straight and even course of the North and South Downs, as well 

 as the gentle undulations of the saddle or dome-like elevation of 

 the western part of our anticlinal line, for the most part denuded 

 of its " tertiary '' covering. But although all the signs of abrupt 

 fracture have disappeared, we still see how the fissures of this 

 stratum have had their edges eroded and spread out, — the 

 deepest into river-valleys, the more superficial into dry trans- 

 Verse valleys and mountain passes on a small scale. A very cur- 

 sory view of the river-courses through the North and South 

 Downs, as they are delineated in the Ordnance Map*, will ex- 

 plain what is here meant, and show how transverse fissures, 

 whether of independent formation, or as the necessary accom- 

 paniments of longitudinal fractures, have been enlarged into 

 valleys by aqueous abrasion. From the chalk we pass to the 

 next rocky stratum, the lower greensand. Here we have more 

 decided evidence of the violence of the denuding operation. 

 There is nothing in the surface arrangement of the chalk that 

 might not be accounted for on the principle of a gradual and 

 gentle removal by sea-currents, or by atmospheric erosion. But 

 a close inspection of the lower greensand escarpment will soon con- 

 vince us, that water in a state of violent and tumultuous agita- 

 tion has been at work immediately consequent on, or in conjunc- 

 tion with, the act of upheaval and the fracture of the rock-masses. 



Of the three groups into which the lower greensand is divided, 

 and each of which has a distinct country, as faithfully and mir 



j * In all matters of local detail the reader is referred to the Ordnance 

 Map ; and a comprehensive notion of the act of denudation will be mainly 

 assisted by the study of the arrangements of its high grounds and escarp- 

 ments. 



