262 KEY TO TERRESTRIAL HISTORY 



escape this acute observer; so that when for the first 

 time he viewed the wolds of Yorkshire from the top of 

 York Minster, he gleefully remarked he could " see at 

 once they were chalk from the form of their contours." 

 Thus, while steadfastly pursuing the main object of his 

 search, he discovered by the way one of the most interest- 

 ing branches of popular geology the connexion between 

 structure and scenery. 



The simplicity in structure which he attributed to the 

 east of England would not appear to the eye of a casual 

 observer ; it is often masked by the irregular manner 

 in which the strata crop out at the surface of the 

 ground. But Smith was a born geometer, and recog- 

 nised very early the effects which the contour of the 

 surface must produce on the outcrop of gently-sloping 

 beds. 



The nature of the sediments may thus prove of great 

 value as a means of identification ; Smith's own work 

 shows this clearly. Yet it is a means open to much sus- 

 picion, only to be employed with the greatest prudence ; 

 for observation shows that the same sheet of sediment 

 may change its character when traced over a considerable 

 area ; while, again, two or more strata of different ages, 

 or belonging to different horizons of the series, may con- 

 sist of the same kind of sediment, so as in this respect to 

 be indistinguishable from each other. If this be true of 

 such a limited area as the British Isles, as it undoubtedly 

 is, how far less trustworthy must the sedimentary 

 character become when applied to the identification of 

 strata in different parts of the world. So greatly have 

 geologists been impressed by this consideration that they 

 have sometimes shown signs of undervaluing the evidence 

 to be drawn from the nature of sediments, and have been 

 led to underestimate its constancy in geographical distri- 

 bution it is difficult to avoid Scylla without running into 



