THE ORIGIN OF SOILS 29 



remains. Limestones and chalk, formed from the accumu- 

 lated remains of certain mollusca and other creatures whose 

 shells consist of carbonate of lime, are probably the most 

 important of those of animal origin. Coal, peat and inter- 

 mediate products composed of the remains of vegetable 

 organisms may also be regarded as rocks from the geo- 

 logical standpoint, and some of them are of considerable 

 agricultural importance. 



Metamorphic Rocks. Sedimentary and organic rocks 

 which have been subjected to great heat and partly changed 

 or metamorphosed, are called metamorphic rocks. 



Stratification. Aqueous and organic rocks, owing to the 

 mode of their formation, are found in layers or strata_, 

 and are called stratified rocks. The others are unstratified. 

 This is usually the primary basis of geological classifica- 

 tion and may be presented in tabular form as follows : 



CLASSIFICATION OF BOCKS. 



I. Unstratified Bocks. 



1. Igneous. 



(a] Plutonic granite. 



(b) Volcanic basalt. 



2. Metamorphic marble. 



II. Stratified Bocks. 

 1. Aqueous. 



( arenaceous sand- 



(a] Sedimentary < stone. 



\ argillaceous clay. 



(6) Solutionary gypsum. 



2. Organic. 



(a) Animal limestone. 

 (&) Vegetable peat. 



The unstratified rocks presently existing are not neces- 

 sarily older than the stratified formations, in fact some 

 of them are quite recent. They consist of crystalline 

 minerals and contain no fossils. Granite and basalt 

 respectively may be taken as typical examples of the 

 two principal sub-divisions of the igneous group. They 

 exhibit, in well marked degree, characteristic differences 



