14 INTRODUCTION. 



sandstone, grits, and most conglomerates and breccias. In the 

 Argillaceous group are those Aqueous Rocks which contain a cer- 

 tain amount of clay or hydrated silicate of alumina. Under this 

 head come clays, shales, marls, clay-slate, and most flags or flag- 

 stones. In nature there exists, it need hardly be said, no rigid line 

 which separates the Arenaceous from the Argillaceous rocks. The 

 two groups are connected together by endless transitional forms, and 

 we must regard all the mechanically-formed rocks as variable mix- 

 tures of different ingredients, their precise character depending on 

 the predominance of some one constituent. 



B. Chemically-formed Rocks. — In this section are comprised 

 all those Aqueous Rocks which have been formed by chemical 

 agencies. Since, however, many of these chemical agencies are 

 exerted through the medium of living beings, whether animals or 

 plants, we get into this section a number of what may be called 

 " organically-formed " rocks. The most important of the Chemi- 

 cally-formed Rocks are the so-called Calcareous Rocks, comprising 

 all those which contain a large proportion of carbonate of lime, or 

 are wholly made up of this substance ; but there are other rocks, 

 of different composition, formed by chemical or organic agency, 

 which may be briefly noticed. 



As an example of a rock the origin of which is purely chemical, 

 we may take rock-salt or sodium chloride, extensive deposits of which 

 occur in formations of all ages, from the Silurian upwards. Whatever 

 may have been the precise mode in which these deposits were 

 formed, it is quite certain that the salt existed, to begin with, in 

 solution in water, and that its assumption of the solid form was the 

 result simply of precipitation. Hence, rock-salt is invariably com- 

 posed of larger or smaller crystals of sodium chloride, though not 

 uncommonly rendered impure by intermixture with sand or clay. 



Another rock which may be regarded as a direct product of 

 chemical action, apart from the operation of living beings, is gypsum 

 or calcium sulphate. This substance, apart from other modes of 

 occurrence, is not uncommonly found interstratified with the ordinary 

 sedimentary rocks, in the form of more or less irregular beds ; and 

 in these cases it has a certain palaeontological importance, as occa- 

 sionally yielding well-preserved fossils. In general appearance, 

 gypsum, when occurring in mass, is usually a whitish, yellowish, 

 or reddish granular rock, which can be easily shown by the micro- 

 scope to be composed of crystals of calcium sulphate. Very com- 

 monly, indeed, the rock is as coarsely crystallised as loaf-sugar, or 

 more so, and the microscope is not needed for the recognition of its 

 true structure. With regard to its mode of origin, there is no reason 

 to doubt that deposits of gypsum are formed by the direct precipita- 

 tion of calcium sulphate from solution in water, without the inter- 



