FORMATION OF ROCKS. 7 



The term Psammitic (from psammos, sand) has been applied to 

 sandy rocks and hard quartzose sandstones ; and Pelitic or 

 Pelolithic {horn pelos, clay) to the muddy and clayey deposits. 



The term Greywacke was formerly applied to the older Primary 

 rocks. Any limestone and some other rocks of ornamental 

 character, such as Alabaster, Serpentine, etc., if capable of being 

 polished, are popularly known as Marble. 



The Aqueous or Sedimentary and Subaerial Rocks may be again 

 classified, according to their method of formation, as follows : — 



1. Mecha7iicanv formed {C\diSi\c,Yxdigmexilci\ ox Detrital Rocks): — 

 Gravel, Sand, Conglomerate, Breccia, Silt (fine sandy mud), Clay, 

 Shale, Loam, Marl, and some Limestones. 



2. Chemically formed: — Ironstone and Cement-stone Nodules, 

 Septaria,^ Rock-salt, Alabaster or Gypsum, Tufa, Chert and Flint 

 (in part), Dolomite, and some Limestones. 



3. Orgaiiically formed: — Peat, Lignite, Coal, Chert and Flint 

 (in part), and most Limestones. 



In all attempts to classify the rocks, we find that hard lines 

 of separation do not exist, as many agencies work together in 

 their formation. Nor in the matter of names can we be 

 always precise, for the rocks pass one into the other ; clay into 

 loam or marl, and these into sandstone or limestone ; and we 

 may often hesitate whether to term a rock calcareous sand- 

 stone or sandy limestone. As Sedgwick long ago exclaimed, 

 " How impossible it is for us to constrain the vast and com- 

 plicated operations of Nature by the fetters of a rigid 

 definition ! " 



Moreover deposits formed by chemical action may be mingled 

 with organic remains and mechanically formed sediment. 

 Coal and other beds formed organically have been modified 

 by chemical action. Some limestones may have resulted from 

 the destruction of older calcareous rocks, having been formed 

 from material carried in suspension in water, and ultimately 

 deposited as a sediment in lake or sea ; while others may be 

 due to precipitation from water holding bi-carbonate of lime 

 in solution. Limestones may also be composed in great part 

 of the minute calcareous tests of Foraminifera, or of the shells 

 or comminuted fragments of other organisms ; or they may 

 be largely due to the secretion of carbonate of lime by Corals. 

 These rocks may be termed of organic origin. But Darwin 

 has observed that in recent coral formations the quantity of 

 stone converted into impalpable mud by the excavations 



' These are nodules of argillaceous limestone or clay-ironstone, divided by 

 septa or cracks filled with mineral matter, usually calc-spar. 



