THE EARTH'S AGE 217 



Hoiu to Estimate the Thickness of the Sedimentary Rocks. 

 ■ — The sedimentary rocks of which the earth's crust is 

 mainly comi^osed consist, according to Sir Charles Lyell's 

 classification, of fourteen great formations, of which the 

 niost ancient is the Laurentian, and the most recent the 

 Post-Tertiary or Pleistocene; with thirty important sub- 

 divisions, each of which again consists of a more or less 

 considerable number of distinct beds or strata. Thus, the 

 Silurian formation is divided into Upper and Lower 

 Silurian, each characterized by a distinct set of fossil 

 remains, and the Upper Silurian again consists of a laro-e 

 number of separate beds, such as the Wenlock Limestone, 

 the Upper Llandovery Sandstone the Lower Llandovery 

 Slates, &c., each usually characterised by a diiference of min- 

 eral composition or mechanical structure, as well as by some 

 peculiar fossils. These beds and formations vary greatly in 

 extent, both above and beneath the surface, and are also of 

 very various thicknesses in different localities. A thick bed 

 or series of beds often thins out in a given direction, and 

 sometimes disappears altogether, so that two beds which 

 were respectively above and beneath it may come into contact. 

 As an example of this thinning out, American geologists 

 adduce the Palaeozoic formations of the Appalachian Moun- 

 tains, which have a total thickness of 42,000 feet, but as 

 they are traced westward thin out till they become only 

 4,000 feet in total thickness. In like manner the Carboni- 

 ferous grits and shales are 18,000 feet thick in Yorkshire 

 and Lancashire, but they thin out southwards, so that in 

 Leicestershire they are only 3,000 feet thick ; and similar 

 phenomena occur in all strata and in every part of the 

 world. It must be observed that this thinning out has 

 nothing to do with denudation (which acts upon the 

 surface of a country so as to produce great irregularities of 

 contour), but is a regular attenuation of the layers of rock, 

 due to a deficiency of sediment in certain directions at the 

 original formation of the deposit. Owing to this thinning 

 out of stratified rocks, they are on the whole of far less 

 extent than is usually supposed. When we see a geologi- 

 cal map showing successive formations following each 

 other in long irregular belts across the country (as is well 



