THE MATERIALS OF STRATA. 37 



grinding up of flints, liberated from the destruc- 

 tion of the chalk. 



Pebble beds frequently mark important divi- 

 sions of geological time in the country in which 

 they are found; because their existence implies 

 that change of level of land which resulted in the 

 tidal denudation which brought them into exist- 

 ence. Among examples of great pebble beds and 

 conglomerates may be mentioned the geological 

 deposit known as the Llandovery beds, which in 

 Tins the base <>f the true Silurian rocks, 

 and extends ssively over the upturned edges 



of the older Cambrian rocks, which had previously 

 been planed level by the sea. 



Sa '. 



There is a rapid gradation between pebble- 

 beds and sand-. Beds of intermediate texture, 

 with many grains " as pea-, are named 



grits, and a 1 in some layers of the 



11 Millstone grit," which underlies the coal. The 



IIS are miniature pebbles, often angular, formed 

 by rounding angular masses of quartz rock on a 



shore. On a sandy shore, like that of Hast- 



5, Reculvers or Hunstanton, the sands now 

 being deposited are derived from sandstones 

 which form the cliffs, broken up first by joints, 

 and afterwards separated into grains, similar to 

 the material out of which such sandstones were 

 originally consolidated. Cambridge green sand 

 and Neocomian sand contain many rock frag- 

 ments and fossils derived from ancient deposits. 



The particles of quartz are often crystalline, 

 but are sometimes derived from uncrystalline 

 forms of silica such as chalcedony, chert, opal, 



