CLASTIC ROCKS 117 



by other means. The material is very fine grained, and agrees in 

 composition and character with the finer marine sediment and 

 that laid down in the central parts of lakes. If the deposit were 

 well laminated a shale would be formed, and if strong pressure were 

 brought to bear on the material it might be converted into a slate. 



All the rocks hitherto described agree in being made up of 

 fragments derived from pre-existing rocks, broken, rounded, or 

 ground down to various stages of fineness. Any of them may 

 have been formed in the sea or by rivers or in lakes, and some 

 may even have been built up on land. The exact method of 

 origin would have to be decided in each case, mainly from the 

 fossils contained in the rock. The majority, on application of 

 this test, prove to have been marine in origin, and the sea may 

 be looked upon as the chief place of origin of the clastic rocks. 



There are other rocks coming into the same category which are 

 not so obviously derived from pre-existing rocks. Chief amongst 

 these are limestones. There are many different sorts of lime- 

 stone. The majority are richly fossiliferous, the fossils being 

 sometimes seen on a fresh fracture; but they are more easily 

 collected from weathered rock, and in some cases are only seen 

 in microscopical slides prepared by grinding thin chips of the 

 rock till they are transparent. The majority of limestones when 

 examined in one of those ways prove to be made of broken frag- 

 ments of organisms provided with a calcareous skeleton or test. 

 Sometimes these are calcareous algae such as Chara or nullipores : 

 More usually they are animals such as foraminifera in the Chalk 

 or Carboniferous Limestone ; corals, crinoids, polyzoa, as at Dudley 

 (Fig. 24) or Wenlock ; sea-urchins, brachiopods (Fig. 26), as in 

 some of the Oolites ; lamellibranchs, gastropods, or cephalopods. 

 Most of them, again, are marine, but there are limestones made of 

 pond snails or land snails, and shell marls made from fresh-water 

 shells in lakes. Some few limestones are clearly made from car- 

 bonate of lime precipitated chemically from springs or in lakes, 

 and this is also probably the origin of some of the magnesian 

 limestones, which are usually very poor in fossils. Thus limestones 

 are rocks made from the carbonate of lime removed by chemical 

 denudation, precipitated either by the agency of life or by evapora- 

 tion of water and loss of carbonic acid. 



