1 8 INTRODUCTION. 



The term Formation, as defined by Lyell, means an assemblage 

 of rocks which have some character in common, whether of origin, 

 age, or composition. Thus we may speak, by way of contrast, 

 of stratified and unstratified formations, and in like manner of 

 freshwater and marine, aqueous and volcanic, ancient and modern, 

 metalliferous and non-metalliferous formations. 



The term Cycle has also been applied to the three great divisions 

 in time of Palaeozoic, Mesozoic, and Ccenozoic. 



No particular restriction can be given to the use of the terms 

 Beds, Rocks, Strata, and Deposits. In many cases, however, it is 

 more convenient to use the term Beds than one indicating a 

 particular lithological character, such as Thanet Beds instead 

 of Thanet Sand ; but in other cases, such terms as Red Crag, 

 Oxford Clay, and Old Red Sandstone are so generally used that 

 they are preferred. 



It is evident that a deposit will be subject to variation 

 according to the conditions of the area in which it was 

 formed ; changing just as any marine or freshwater accumu- 

 lation changes at the present day from sand to shingle or 

 mud. Conglomerates, for instance, may occur at all horizons 

 on or near the margin of a series : as in the case of the Trias. ^ 

 Hence were our marine formations complete, so far as they 

 were originally deposited, we might expect to find evidence 

 of shore accumulations in all cases as well as the deposits of 

 deeper water. The results of denudation, and the limited 

 areas over which many formations are exposed, prevent our 

 seeing more than a portion of each. So that when we speak 

 of one formation representing deep-water conditions, and 

 another shallow-water or terrestrial conditions, it must be 

 remembered that the records only of particular areas in the 

 physical history of each formation are, as a rule, presented to 

 our view. It is generally admitted that no good classification 

 of strata can be made until the sequence of rocks has been 

 ascertained ; that " to begin with fossils, before the physical 

 groups are determined, and through them to establish the 

 nomenclature of a system, would be to invert the whole logic 

 of geology, and could produce nothing but confusion and 

 incongruity of language." ^ And yet the history of geological 

 research proves how impossible it is to select any one locality 

 as " typical." The rocks change, and the assemblages of 

 fossils change also. Strictly speaking the names of our 

 formations should be the expression of certain physical con- 

 ditions ; but as these conditions were, in all probability, 



' See remarks on conglomerates by A. Geikie, QJ. xxxix. 305. 

 - Sedgwick, Q.J. viii. 7. 



