10 INTRODUCTION, 



having been formed at different depths, or in distinct 

 zoological provinces."^ Further, in studying the fossils of any 

 particular formation, we must expect an admixture of marine 

 forms that could not have existed together in the same depth 

 of water, as forms which frequent shallow water would some- 

 times be drifted into deeper sea, and animals belonging to 

 the deeper sea are occasionally washed on shore. 



Bivalve Mollusca embedded in their natural positions are 

 important guides in ascertaining the depth of water during 

 the deposit of a stratum. So also may be the occurrence of 

 footprints of animals, their tracks or burrows, the impres- 

 sions of rain-drops, ripple-marks, sun-cracks, or false-bedding. 

 Nevertheless, deposits that originally contained many fossils 

 have sometimes lost all traces of organic remains by chemical 

 agency.^ 



The rocks of England and Wales belong mostly to the 

 Sedimentary or Stratified group, and they constitute a series, 

 of which the subdivisions are distinguished by certain promi- 

 nent lithological characters, and often by the presence of par- 

 ticular fossils. Moreover, the newer the rocks in which these 

 organic remains are embedded, the more closely are the fossils 

 allied to the forms of life now in existence — a doctrine first 

 taught by Werner. It is true that we have clay, limestone 

 and marl, sandstone, sand and conglomerate, intercalated one 

 with another, at all horizons in this great series ; but the 

 sequence of the groups of rocks has been clearly established 

 by observation. The relations of the different strata may 

 be seen in many cliffs, in quarries, brickyards and railway- 

 cuttings f while the order is confirmed by the records of deep 

 mines and wells. When such evidence is wanting in par- 

 ticular districts, the stratigraphical position of the rocks may 

 be determined by tracing out their extension across country, 

 and noticing their relation to the form of the ground ; when 

 it will be perceived that the lithological characters of the 

 rocks exercise a marked influence on the scenery, and in 

 ascending a hill the soils, springs, and vegetation may indicate 

 the superposition of distinct strata. Observations of this 



^ See the Mamtal of Molhcsca, by S. P. Woodward, p. 410 ; and J. Morris, P. 

 Geol. Assoc, v. 198. 



- See remarks on the dissohition of shells from sand, by W. Whitaker, Q. J. 

 xxxiii. 122 ; and on disappearance of fossils from clays, by Dr. P. M. Duncan, 

 Ibid. xxii. 12. See also H. C. Sorby, Address to Geol. Soc. 1879, p. 65. 



^ The term Section in Geology is applied to any artificial excavation, boring or 

 cutting, or natural cliff or bank, in which some geological stratum or set of strata 

 has been exposed or proved. 



