THE MATERIALS OF MOUNTAIN CHAINS. 2 1 



cleavage causes it to split in layers which cut 

 across the original folded or faulted planes of 

 bedding. This microscopic crystalline change of 

 texture imparts to the rock, now termed slate, a 

 remarkable durability. Its particles are laced 

 together by a network of parallel films of micro- 

 scopic crystals. Slates may be of any antiquity. 

 N thing but folding and uplifting of mountainous 

 ma^o is needed to form them. In England and 

 America they belong chiefly to the ancient epochs 

 of time distinguished as pre-Cambrian, Cambrian, 

 Silurian, and 1 )evonian. 



-The transitions between slate and 

 schist arc common in mountain regions. Crystals 

 Of other minerals are sometimes developed on the 

 je planes of slate. Some slates are very 

 micaceous; and it is sometimes difficult to say 

 where mica slate ends, and mica schist begins, 

 The original bedding is usually obliterated in 

 schists; so that the rocks give no evidence of 



having been deposited in water. Occasionallyi 



as in the mica slates south of Bergen in Norway, 

 beds of limestone, in which fossils are preserved, 

 are found in such rocks. In the north of Scot- 

 land fossil-bearing beds, known as the Durness 

 limestone, occur between schists, where they are 

 introduced by horizontal dislocations. 



A schist presents to the eye an arrangement of 

 short irregular layers of crystals, which is similar 

 to the appearance which a thin film of slate shows 

 under the microscope, although schists differ from 

 slates in having all their material crystalline. 

 There is some reason for regarding them as re- 

 sults of intenser action of such compression as 

 imparted a slaty texture to ancient beds of very 

 varied mineral character. 



