THE MATERIALS OF STRATA. 31 



iblish a transition from the plutonic rocks 

 through schists to water-formed deposits. It has 



n»»t been fully collected, but deserves examina- 

 tion, the earth offers no indication (A a be- 

 lling in its j al history. If metamor- 

 phism Mich as is manifest in the older rocks were 

 ended over the earth's surface it would oblit- 

 erate record-. And the wearing up oi such 



metamorphosed rocks into new sediments would 



ensure a succession of similar rock materials. 



CHAPTER Y. 



THE MATERIA] (RATA. 



Terrestrial Kchks. 



AROUND many parts n{ the roast, as in Lanca- 

 shire and Norfolk, the winds blow up sands from 

 the sea-bed, laid bare at low tide. These sands 

 form low ranges of hills, known as sand dunes. 

 They often show forms of hill contours as varied 



ire produced by the work of water and frost in 

 Carving hills out of solid material. These sand 

 dunes are but an insignificant illustration of the 

 work done by the wind, in heaping' and rounding 

 the grains of sand which form desert regions. 

 There, every grain of quartz, which in a sandstone 

 usually retains some of its angles of crystal form, 

 is rounded by long continued motion, till it be- 

 comes a miniature pebble. There is some evi- 

 dence that desert conditions not altogether dis- 

 similar to those of Arabia or the Sahara may 

 have existed in Great Britain at the beginning of 

 the Secondary period of time, when the rock salt- 



