LITHOLOGICAL CHARACTERS 261 



use of by Werner, and was employed by Smith himself. 

 Thus by its colour, texture, and composition he tracked 

 the red marl, which we now assign to the Trias, across a 

 great part of the length and breadth of England ; so by 

 its alternating layers of thin, hard limestone and blue 

 clay he followed the Lias. Indeed, by this means alone 

 he gained a considerable insight into the structure of the 

 eastern half of England. The red marl he represented 

 as lying unconformably upon the older coal-measures ; 

 it forms the fertile plains of central England, and dis- 

 appears to the east beneath the thin limestones and clays 

 of the Lias (Fig. 82) ; these also slope gently to the east, 

 and similarly disappear beneath the Oolites, which again 



Snowdon 



FIG. 82. Section across England and Wales. 1. Pre-Cambrian Eocks. 

 3. Ordovician. 4. Silurian. 5. Old Bed Sandstone. 8. and 9. 

 The "Bed marl" or Trias. 10. Lias. 11. Oolites. 12 and 13. 

 " Greeiisand and Chalk " Cretaceous. 14. Eocene. f f . A fault. 

 s.l. Sea-level. 



slope eastwards and disappear below the greensand and 

 chalk. 



Here we may briefly refer, in passing, to a discovery 

 which lies outside our immediate problem. The strata 

 which we have just enumerated are of differing degrees 

 of hardness, and Smith connected this difference with the 

 contour of the land, pointing out that the softer beds 

 form low plains ; the harder, ranges of hills which face 

 with a comparatively steep scarp towards the west; while 

 on the east they very gently slope away, to disappear 

 beneath succeeding strata. This difference in relief is 

 a consequence of the unequal resistance of the beds to 

 the wearing action of rain and rivers. Even the subtleties 

 of form, which we call the modelling of the hills, did not 



