20 THE CRUST WE DWELL UPON. 



ever the structure of the crust is revealed whether along 

 the cliffs of the sea- shore, in ravines worn out by rivers, in 

 shafts sunk for mining, or in railway- cuttings and tunnels 

 we see the rock-masses arranged in two great ways. A 

 large and extensive class like the sandstones and shales 

 and limestones lie in layers or beds one above another; and 

 a second class like the granites, greenstones, and basalts 

 exhibit no lines of bedding or layers, but occur in vast and 

 indeterminate masses. These two modes of arrangement 

 may be seen in almost every railway-cutting and sea-cliff, 

 and must obviously have arisen from different causes. Now 

 the great maxim in geology is to reason from the known to 

 the unknown, and to appeal from the existing operations of 

 nature to the operations of the past. For, as was long ago 

 well remarked by Hutton, " when from a thing which is 

 well known we explain another which is less so, we then 

 investigate nature ; but when we imagine things without a 

 pattern or example in nature, instead of Natural History, 

 we write merely fable." Abiding by this method, we find 

 nature at the present day laying down, in every lake and 

 estuary and sea, layers of mud and sand and gravel, vary- 

 ing in thickness and continuity, according to the extent 

 of the areas and the magnitude of the in-flowing rivers; 

 and were these layers consolidated, sand would form sand- 

 stone, gravel conglomerate, and mud shale. Here then, 

 as we cannot regard nature acting in tune past otherwise 

 than at present, we are entitled to infer that all rocks in 

 the earth's crust occurring in layers have been formed 

 through and by the agency of water that is, that they 

 are the sediments of former lakes and estuaries and seas, 

 the particles of which they are composed having been worn 

 down by water, transported by water, and deposited in 

 water. Hence all such rocks are regarded as aqueous, sedi- 

 mentary, or stratified, and indicate that the areas they now 



