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 

 propriety, belong to this section. It is agreed to admit roofing 

 slate, slate clay, or shale, transition limestone, and greywacke. 

 Mr. Bakewell adds the old red sandstone, as belonging to the 

 greywacke series : he also includes the mountain limestone, as 

 an upper portion of the transition limestone. The first two 

 are included in Mr. Coneybeare'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 comparative 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 order, and for the purpose of more convenient 

 classification, most geologists have subdivided the series. 



The Lower Series 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 



