n8 THE BOOK OF NATURE STUDY 



Associated with the last class of rocks are the rock-salts and 

 gypsums, which have also been formed by precipitation occurring 

 probably in inland salt lakes. 



The last of the more important groups of clastic rocks to be 

 considered are those in which carbon is an important constituent. 

 The only important deposit of such a character forming now is 

 peat, which is the result of the growth of numerous bog plants in a 

 damp climate. The plants grow and die down, and newer plants 

 grow on their surfaces ; the dead plants lose some of their gaseous 

 constituents and slowly pass into peat. Coal seams in like 

 manner are made of plant remains, which have probably grown 

 where they are found, and their harder and more protective parts 

 have gradually accumulated. On being buried up by later 

 sediments they have become compressed and hardened, have 

 lost some of their volatile constituents, become bituminous and 

 mineralised, and passed first into lignite, then into bituminous 

 coal, and lastly into steam-coal and anthracite. 



There are, of course, many minor varieties of rocks and many 

 which stand intermediate between the forms described, but those 

 mentioned are typical of their classes, and most of the known 

 clastic rocks belong to one or other of the divisions mentioned, 

 namely, pebbly rocks, sands, clays, calcareous rocks, and carbon- 

 aceous rocks. The crystalline rocks will be considered in the next 

 chapter. 



When we come to compare each of the rocks with the sediment 

 that it most resembles we at once find important differences 

 which will be considered later. But already important resemblances 

 will have been seen in the examination of hard specimens, and other 

 points of resemblance come out when the mode of occurrence of 

 the rocks in natural or artificial excavations comes to be studied. 



The solid framework of the land can be seen in any kind of 

 excavation which has been carried beneath the soil and subsoil. 

 Railway or road cuttings, wells, streams, mines, and quarries, are 

 the sections which give us our information as to the structure and 

 composition of this rock framework. Any of these kinds of 

 exposure which may happen to be available should be next 

 studied. In the majority of them the rocks will be found arranged 

 in layers like those of gravel and alluvium, only the layers will 



