1 70 ELEMENTS OF SCIENCE 



prodigious glaciers,* which appear to have scooped out 

 valleys, and scored the surface of hill and dale in 

 those regions. Blocks of stone or " boulders " are often 

 found scattered about there, and seem to have been 

 transported by ice, sometimes for very great distances. 



The various strata which thus form the crust of the 

 earth contain, in different degrees of rarity or abundance, 

 certain objects which are known as "fossils." Amongst 

 the mass of materials carried down by rivers and 

 deposited along their course or in their deltas, or at the 

 bottom of the sea, are numerous relics of that kind. 

 When such a relic becomes entombed, it often happens 

 that particle by particle of its substance is replaced, 

 particle by particle, by mineral matter (ferruginous, 

 calcareous, or silicious) till we have a complete represen- 

 tation technically called a " pseudomorph" of the 

 original in such new material. 



There are five kinds of fossils : 



(i) Objects preserved unchanged, or little changed so 

 that they retain the greater part of their own mineral 

 material matter; (2) substitutes or "pseudomorphs"; (3) 

 moulds, i.e., deposits which present the impressions made 

 by objects, all other evidences of which have disappeared. 

 Thus impressions of hailstones made in a soft surface of 

 mud, have often been preserved by subsequent delicate 

 layers of deposit. Then, the whole having become 

 hardened, the shape of such impressions when laid bare 

 by the geologist, may show us to-day the direction in 

 which the wind blew when those hailstones fell at some 

 unimaginably distant period of past time ; (4) casts of 

 moulds, i.e., solid matter which has taken the place of 

 whatever bodies may have produced the moulds and then 



* See ante, p. 159. 



