192 THE BOOK OF NATURE STUDY 



deposits of blown-sand, alluvium, or glacial debris, all of which 

 will typically contain land animals and plants alone, or no fossils 

 at all. 



When fossils are collected the nature of the containing rock 

 should be noted in order to see whether it confirms or contra- 

 dicts the conclusions drawn from the fossils. Pelagic forms are 

 usually found in clays or limestones ; shore-living forms are 

 associated with coarser grained deposits ; terrestrial and fresh- 

 water forms not often with limestones, unless these have been 

 deposited in tufaceous form from springs. Seams of coal and 

 lignite will be found resting on old alluvial soils, penetrated by 

 rootlets, and containing only land and fresh-water forms ; fossil 

 footprints, usually with ripple-marked sandstones, made up 

 of rounded grains, and associated with thin clay beds. Evidences 

 of shallow-water deposits are easily detected by ripple marking, 

 tracks of worms and other animals (Fig. 20), false bedding, and 

 the arrangement of the coarser materials (Figs. 17 and 24) ; while 

 terrestrial deposits are usually tumultuous and unsorted (Fig. 22), 

 and singularly barren of fossils. 



The units of geological history are those strata which are 

 of sufficient individuality and importance to be called Formations. 

 No better example can be chosen than the Chalk, a pure, earthy, 

 white limestone about 1000 feet thick (Fig. 66), easily recognised 

 and retaining its characters not only all over England, but through- 

 out Northern Europe. It would be a justifiable inference that 

 it is approximately contemporaneous in origin throughout its 

 extent, and that beds immediately preceding and succeeding it 

 in England would be of the same age as those next below and 

 above it in Russia. There are many similar Formations in 

 British geology, and it will be sufficient to remark the Old Red 

 Sandstone, the New Red Sandstone, the Coal Measures, the 

 Mountain Limestone, the Great Oolite, and the London Clay. 



Originally the geological record was pieced together in single 

 districts or countries, and it was then sufficient to find the order 

 of conspicuous Formations and to discover the nature and 

 thickness of the less conspicuous Formations in between them. 

 Gradually it was discovered that Formations were not so sharply 

 marked off from one another as they at first seemed to be, but 



