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 Palaeozoic 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 
