STRATIFICATION. 91 



in the Connecticut valley. They are the prevailing rocks of Bri- 

 tain, including within their series the chalk, oolite, coal strata, and 

 others. They occur over nearly all Europe, spread throughout the 

 great plains of Eussia, through Asia nearly to the top of the Hima- 

 layas, South America in many places to the summit of the Andes, 

 and through Africa and Australia. These stratified rocks are in strik- 

 ing contrast with the unstratified, — granite, for example, which may 

 show no appearance of layers even through heights of a thousand 

 feet or more. Many volcanic masses of rock are unstratified. Yet 

 the volcanic mountain has usually a stratified arrangement, successive 

 layers of lava and volcanic sand or earth being piled up to make 

 the cone. Even among crystalline rocks the distinction of strata 

 may often be made out, although much disguised by changes in 

 the course of their history. 



The succession of strata in stratified rocks is exceedingly various. 

 In the section given, there are alternations of limestones, shales, 

 and sandstone. In others, as at Trenton Falls, N.Y., there are 

 only limestones in sight ; but were the rocks in view to a much 

 greater depth, sandstone strata would be seen. In still other 

 regions, there are alternations of conglomerates and shales ; or 

 conglomerates with shales and coal-beds ; or conglomerates with 

 limestones and sandstones ; or shales and sandstones alone. 



The thickness of each stratum also varies much, being but a few 

 feet in some cases, and hundreds of feet in others ; and the same 

 stratum may change in a few miles from 100 feet to 10, or disap- 

 pear altogether. In the Coal formation of Nova Scotia there are 

 14,000 feet of stratified beds, consisting of a series of strata mainly 

 of sandstones, shales, and conglomerates, with some beds of coal ; 

 and in the Coal formation of Pennsylvania there are 6000 to 7000 

 feet of similar character. 



98. After these illustrations, the following definitions will be 

 understood. 



a. Stratification. — A succession of rock-layers, either of the same 

 or of different kinds. 



b. A layer. — A single member or bed in a stratified rock. It may 

 be thick or thin, and loosely or strongly attached to the adjoining 

 layers. In the section, fig. 60, the limestones 4 and 6 consist of a 

 great number of layers; and in all limestone regions many are 

 piled together to make the great mass of limestone. 



c. A stratum. — The collection of layers of one kind which form a 

 rock as it lies between beds of other kinds. In the section re- 

 ferred to (fig. 60), the limestones 4, 6, and the shale masses 3, 5, 7, 

 are each a stratum. A stratum may consist of many layers. 



