CH. in] ROCK-BUILDING. 23 



well expressed by Freeman, in speaking of architectural 

 styles, — "Our minds," he says, "are more used to definite 

 periods ; they neglect or forget transitions which do indeed 

 exist'." The idea of definite classification is liable to narrow 

 our view of uniformity and the natural sequence of events. 



Composing that part of the earth which is accessible to 

 us, — or as it is generally called the earth's crust, — there are 

 rocks of various kinds, of which some have been formed by 

 igneous agency, either as lavas or beds of ashes, or in the 

 form of molten magmas which gradually cooled and became 

 crystalline below a mass of superincumbent strata. With 

 these rocks we need not concern ourselves. 



A large portion of the earth's crust consists of such 

 materials as sandstones, limestones, shales, and similar strata 

 which have been formed in precisely the same manner as 

 deposits are being accumulated at the present day. The 

 whole surface of the earth is continually exposed to the 

 action of destructive agencies, and suffers perpetual decay ; it 

 is the products of this ceaseless wear and tear that form the 

 building materials of new deposits. 



The operation of water in its various forms, of wind, 

 changes of temperature, and other agents of destruction 

 cannot be fully dealt with in this short summary. 



A river flowing to the sea or emptying itself into an inland 

 lake, carries its burden of gravel, sand, and mud, and sooner or 

 later, as the rate of flow slackens, it deposits the materials 

 in the river-bed or on the floor of the sea or lake. 



Fragments of rock, chipped off by wedges of ice, or 

 detached in other ways from the parent mass, find their way 

 to the mountain streams, and if not too heavy are conveyed 

 to the main river, where the larger pieces come to rest as more 

 or less rounded pebbles. Such water-worn rocks accumulate 

 in the quieter reaches of a swiftly flowing river, or are thrown 

 down at the head of the river's delta. If such a deposit of 

 loose water-worn material became cemented together either 

 by the consolidating action of some solution percolating 

 through the general mass, or by the pressure of overlying 

 1 W. E. W. Stephens, Life of Freeman, p. 182, London, 1895. 



