8 



" The first sedimentary rocks were formed from the wearing away 

 and breaking up ot the crust by the action of water and of the 

 atmosphere : the disintegrated quartz would be sorted into beds of 

 various degrees of fineness, forming the first sandstones and grits, 

 whilst the carbonic acid would unite with the alkalies of the silicates, 

 and reaching on the earthy and metallic chlorides, which we have 

 already seen were present in large qiiantities in the ocean, would 

 give rise to sedimentary argillaceous deposits, many of which, more 

 or less mixed with silica, were afterwards altered by metamorphic 

 action : of these metamorphic beds, roofing slate is a sufficiently 

 familiar example." * 



Just as the first sedimentary beds were formed from the debris 

 of the original cnist by the action of rivers and of the sea, so were 

 the later beds formed, and are to this day being formed, from the 

 debris of pre-existing beds. In addition to the sands, sandstones, 

 clays, slates, granites, &c., the formation of which we have already 

 glanced at, we have immense beds of limestone, which are for the 

 most part of organic origin, due to the lime-secreting powers of 

 organisms of the lowest types, and which are able, like vegetables, 

 to draw their nourishment in great part from the mineral kingdom 

 alone. Encrinital limestone, for instance, which is made up of the 

 calcareous skeletons of lily-like echinoderms ; the coral growths, 

 the work of lime-secreting zoophytes, not only to be found fossil 

 sometimes in masses 15 feet thick, but even now giving rise to 

 large formations and islands : the chalk beds, too, many hundred 

 feet in thickness, consisting almost entirely of the calcareous 

 remains of foraminifera and the grey chalk sediment now being 

 formed at the bottom of the Atlantic, 95 per cent, of which 

 consists of the very same forms of foraminifera which we find in 

 the chalk of our secondary beds. 



It is probable that the earlier limestones were brought about 

 through the development of organic life by the abstraction of the 

 lime contained in the ocean, and the debris of these in turn would 

 give rise to others. There must have been, however, limestones of 

 true chemical origin which might have resulted in a variety of 



• David Forbes, F.R.S. 



