570 



SCIENTIFIC RECREATIONS. 



Fig. 643. Chalk cliff. 



to the ocean, and absorbed by the crinoideans and molluscs which produced 

 shells. These shells hardened and crystallized became limestone, and whole 

 mountains are formed of this " organic " rock, which is used for so many 

 purposes. 



We have spoken of ORGANIC ROCKS, but there are others, and we 



ought, perha.ps, to have spoken of that 

 kind before the chalk put them aside. 

 Let us go back to our sandy shore again 

 and look at the SEDIMENTARY ROCKS, 

 which are the very first formation. We 

 have all seen sandstone, and visitors to 

 the South Devon Coast will remember 

 the red cliffs near Dawlish and Teign- 

 mouth. These are red sandstone not 

 the very " old red " so pleasantly written 

 of by Hugh Miller, but at any rate sand- 

 stone, and composed of grains of sand. 

 When we were at Dawlish last year a 

 piece of the sandstone had fallen on to 

 the beach, and when the waves came up 

 that stone was no doubt gradually washed away into sand, and then fell to 

 the earth as sediment. 



We said something a few pages back about the wear and tear which is 

 always going on : the mountain is worn away a mass falls, it is broken into 

 smaller pieces; these are carried by a river; the mud is deposited, and the 

 finer particles are ground and rounded into gravel, and finally sand. Beneath 

 the current of the river, and at the bottom of a lake or sea, these sediments 

 (mud, etc.) accumulate one on the top of the other in regular series called 

 strata, and then the weight and pressure acting with the soluble mineral 

 deposits always washing down, consolidate and bind the loose sand-grains 

 into stone, which, in the course of ages, hardens. The stones thus formed 

 from sediments such as gravel, mud, and sand, are termed Sedimentary Rocks ; 

 they have become rocks by enormous and continuous pressure. Thus : 



Sands have become " Sandstones " ; 



Gravel has become " Pudding-stone " (Conglomerate) ; 



Mud and clay have become " Shale " ; 



Calcareous deposits have become " Limestone " ; 



Vegetable deposits have become " Coal." 



So we have sandy, clayey, limy, flinty, and corally rocks under long 

 names respectively Arenarious, Argillaceous, Calcareous, Silicious; and 

 we may add Bitumenous and Ferruginous Irony Rocks to the list. 



Speaking of sediments, it is curious to note the different colours of the 

 Arve and the Rhone which meet near Geneva. The white sedimentary 

 Arve can be traced for a long distance beside, not mixing with, the blue 



