62 



FORMATIONS. 



if occupied by clay and soft materials, a clay dike. 

 How these fissures came to be filled by sucii sub- 

 stances will be a subject of inquiry hereafter. They 

 vary in thickness from a few mches to hundreds of 

 feet. 



When a series of strata of a similar rock are ar- 

 ranged with occasional intervening strata of rocks 

 of another kind, which recur in different parts of 

 the series, they are regarded as having been all 

 formed nearly at the same epoch and under similar 

 circumstances; and such beds are called by geolo- 

 gists formations. Thus the strata of shale, sand- 

 stone, and ironstone that accompany beds of coal, 

 are called the coal formation. Strata of different 

 kinds, in which a gradation is observed into each 

 other, and which contain similar species of organic 

 remains, also constitute a geological formation. The 

 chalk with flints, the lower chalk without flints, the 

 chalk marl, and the green sand under the chaik, are 

 regarded as members of what is denominated the 

 clialk formation.^^ There is one circumstance of 

 frequent occurrence which is apt to confound the 

 young geologist, and that is, in travelling in mount- 

 ainous districts, after passing over a certain series 

 i of rocks, he again meets with them in a contrary 



order. How this is accounted for figure 15 will help 

 to explain. 



Fig. 15. 



C c are parts of the lowest stratified rocks, a a oi 

 the highest Now suppose them to be lying in a 



• 6ak»w«ll's Geology, 



