66 Introduction to Geology. 
very abundantly. Some of them are of mechanical origin, or 
are composed of fragments which previously existed in a dif- 
ferent state. They are more widely extended than the pri- 
mitive rocks, and are “the repositories of the most useful 
treasures of the mineral kingdom.” ‘They are stratified ; their 
beds are commonly highly inclined, dipping in all directions, 
and abounding in singular contortions. 
Geologists “differ as to the number of rocks which, with 
pr opriety, belong to this section. It is agreed to admit roofing 
slate, slate clay, or shale, transition limcorane, and er eywacke. 
Mr. Bakewell adds the old red sandstone, as belonging to the 
ereywacke series : he also erie the mountain limestone, as 
an upper portion of the transition limestone. ‘The first two 
are included in Mr. ne beare’s carboniferous order. 
Nearly the whole of Wales is composed of this series, and, 
in Caernarvonshire, it attains an elevation of more than 3500 ft. 
Secondary. 
The Secondary Rocks comprise a very numerous order, 
abounding in extraneous fossils. These remains are, strictly 
speaking, lapidified, and are generally penetrated with the 
stony matter of their matrix ; often, however, in a finer form, 
so as to admit a crystalline arrangement of its particles. This 
observation is derived from Mr. Coneybeare, in drawing a 
distinction between the tertiary or superior order, where, in 
England at least, this change has not been so effected. Rocks 
of this order “ supply mankind with those important neces- 
saries of life, coal, iron, lead, rock-salt, &c.; besides the less 
valuable articles, marble, freestone, lime, gypsum, plumbago.” 
Secondary rocks are distinctly stratified. 
On the Continent these formations attain a much greater 
elevation than in Britain. They occupy the summits of the 
Pyrenees, and form the upper part of the northern chain of 
the Alps, where they rise to the height of 7000 or 8000 feet. 
In the present imperfect state of our acquaintance with the 
geology of the globe, it is not practicable to ascertain the 
exact con iparative areas occupied by the great divisions of 
rocks, but it is computed that the secondary and tertiary beds 
cover two thirds of the land. 
In consequence of some peculiar characters in the lower 
members of this or der, and for the purpose of more convenient 
classification, most geologists have subdivided the series. 
The Lower Secs comprises the coal measures, millstone, 
grit, and shale, the mountain limestone, and the old red sand- 
stone. These formations are often highly inclined, and abound 
in contortions, curvatures, and faults, denoting the influence 
