: 



8 THE CHAIN OF LIFE. 



deposits of the crust of the earth at more than 70,000 feet, 

 though they cannot penetrate it perpendicularly t more than 

 a fraction of that depth. The two sections, Figs. 6 and 7, 

 showing the seciuence of beds in luigland and in the northern 

 part of Nortli America, will serve, if studied by the reader, to 

 show how, by merely travelling over the surface and measuring 

 the upturned edges of beds, many thousan:ls of feet of deposits 

 may be observed, and their relative ages distinctly ascertained. 



In studying any extensive section of rock we find that its 

 members may more or less readily be separated into distinct 

 groups. Sometimes these are distinguished by what is termed 

 unconformability, that is, the lower series has been disturbed 

 or inclined before the upper has been deposited upon it. Tiiis 

 is seen on a grand scale in the section Fig. 7, in the case 'of 

 the Laurentian and Cambrian formations, and on a smaller 

 scale in Fig. 8 in the unconformable superposition of Devonian 

 conglomerate or Silurian slates at St. Abb's Head. In the last 

 section it is quite evident that the beds of the lower series 

 have been bent into abrupt folds and worn away to a con- 

 siderable extent before the deposition of the overlying series. 

 In such r :ase we know not merely that the upper series is 

 newer than the lower, but that some considerable " time must 

 have elapsed after the deposition of the one before the other 

 was laid down ; and we are not surprised to find that the 

 fossils in the groups thus unconformable to each other are very 

 different. 



But even when the beds are conformable, they can usually 

 be separated into groups, depending upon differences of mineral 

 charactrer, or changes which have occurred in the mode of 

 deposition. One group of beds, for example, may be largely 

 composed of limestone, another of sandstone or shale. One 

 group may be distinguished by containing some special mineral, 

 as, for example, rock salt or coal, while others may be destitute 

 of such special minerals. One group may show by its fossils 

 that it was deposited in the sea, others may be estuarine or 



