THE STORY OF THE EARTH. 



and flint. Under the microscope the source of 

 the grains can usually be recognised; for if the 



quart! • finally crystalline all the crystal 



68 are rarely lost, and the mineral may include 

 hair-like Crystals of other minerals, or minute 

 Cavities in which there may be fluid with a bubble 

 ot' air. This IS enough to show that the quartz 

 came originally from the wearing away of schists 

 or granitic rocks, in times when the level of the 

 land caused those rocks to be exposed to the de- 

 stroying influence of the waves. But although a 

 Sandstone is mainly formed of quarts, it rarely 

 contains more than from 50 to 85 per Cent, of 

 silica — the whole of which is not in the crystalline 

 form of quartz. There are frequently in a sand 

 if water-worn felspar. The felspar, which 

 LS a silicate Of alumina combined with a silicate of 

 I or potash, may decompose, liberating the 

 soluble silicate of soda or potash. Felspar crys- 

 tals abound in the old Cambrian sandstones of 



Barmouth, in the Devonian sandstones of South 



Devon, and in the Trias sandstones of the South 



of England. Sandstones often contain scales of 



the mineral mica, as in the Yorkshire sandst< 



used for paving. Sometimes the □ 



then the iron oxide which was one of its constitu- 

 ent- colour to the sandstone. Many 



IstOneS arc mainly the grains of quartz, which 



tituents ystalline rocks, liber- 



I by the decay of minerals with which 1 



1, and left behind comparatively 



: to the :i w hich they were den , 



The liner particles associated with them which 



ted from the decomposition <»f other mine 

 have been ( a: : led * ' distain CS. A if nam 



Bowing three miles an hour at its bottom 



