210 



ISLAND LIFE. 



[part I, 



Silurian formation is divided into Upper and Lower Silurian, 

 each characterised by a distinct set of fossil remains, and the 

 Upper Silurian again consists of a large number of separate 

 beds, such as the Wenlock Limestone, the Upper Llandovery 

 Sandstone, the Lower Llandovery Slates, &c., each usually 

 characterised by a difference of mineral composition or me- 

 chanical 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 Palseozoic formations of the 

 Appalachian Mountains, 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 Leicester- 

 shire 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 denuda- 

 tion (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 geological 

 map showing -successive formations following each other in long 

 irregular belts across the country (as is well seen in the case of 

 the Secondary rocks of England), and a corresponding section 

 showing each bed dipping beneath its predecessor, we are apt to 

 imagine that beneath the uppermost bed we should find all the 

 others following in succession like the coats of an onion. But 

 this is far from being the case, and a remarkable proof of the 

 narrow limitation of these formations has been recently obtained 

 by a boring at Ware through the Chalk and Gault Clay, which 

 latter immediately rests on the Upper Silurian Wenlock 



