THE NON-LIVING WORLD 165 



them, such superimposed masses being called strata. 

 These strata consist of various, generally more or less 

 horizontal, layers of different materials, and are generally 

 composed of consolidated mud which has been deposited 

 (in the way described) in fresh or salt water lakes, or in 

 deep or shallow seas. But not all rocks are due to the 

 agency of water. Many masses have been ejected in a 

 molten state from volcanoes, and solidified either on the 

 land's surface or beneath the sea, and therefore, in the 

 latter case, under great pressure. Rocks which are thus 

 due to volcanic agency are called igneous rocks. Those 

 of them which have been formed under the sea are called 

 plutonic ; otherwise they are termed volcanic. Igneous 

 rocks are not generally stratified, and they may be of all 

 ages. Some, like those which form part of Snowdon 

 and Cader Idris, are very old. Others, like those of 

 Etna, and those which cover Herculaneurn, are relatively 

 quite recent. Deposits may have undergone four kinds 

 of change : (i) they may have undergone a mere process 

 of drying (as with sands) ; or (2) drying accompanied by 

 pressure (as with sandstone) ; or (3) with chemical action 

 in addition (as with highly crystalline rocks like that 

 called gneiss*) ; or (4) a change may have been produced 

 by infiltration. Thus rocks may be infiltrated by iron, 

 lime, or silica, producing ferruginous, calcareous, and 

 silicious sandstones and conglomerates, which last are 

 sometimes called " pudding stones," and consist of frag- 

 ments of rock cemented together. 



The strata thus forming the crust of the earth are 

 supposed to be from sixteen to eighteen miles thick ; but 

 no boring has yet extended to even one mile in depth, 

 and indeed has scarcely exceeded 3000 feet. The total 



* See ante, p. 147. 



