2go PRINCIPLES OF STRATIGRAPHY 



Composition of Clastic Rocks. 



In composition clastic rocks may be pure or impure, simple or 

 complex. If one mineral type predominates, such as lime or quartz, 

 this fact may be combined with the corresponding textural term 

 into a compound word expressive of both. Thus, if the material of 

 the clastic rock is pure lime, the rock becomes a calcirudytc, a cal- 

 carenyte or a calcilutyte, according to the texture. If quartz, the 

 rock becomes a silicirudyte, a silicarcnyte, or a silicilutyte, accord- 

 ing to the texture. If the rock is impure it will still be possible to 

 designate it, keeping in mind that the dominant mineral constitu- 

 ent furnishes the name. Thus we may have siliceous calcirudytes, 

 calcarenytes, calcilutytes or calcareous silicirudytes, silicarenytes, 

 silicilutytes. Or the impurities may be iron, carbon, clay, etc., in 

 which case we use the prefixes ferruginous, carbonaceous, argil- 

 laceous, etc. 



Lutytes are most generally formed among the argillaceous or 

 clay rocks, but pure argillutytes are not very common. Generally 

 they are siliceous, calcareous or carbonaceous argillutytes, all of 

 which are more familiarly known by the structural terms shales or 

 slates, which terms, however, express nothing definite in regard to 

 the composition. 



Clastic rocks of complex composition, as, for example, those 

 formed from the reconsolidation of disintegrated granites (ark- 

 oses), can be spoken of simply as rudytes, arenytcs, or lutytes, 

 without attempt at defining their composition. 



Examples of elastics under each group are as follows : 



I. The Pyroclastics. In composition these are seldom simple, 

 being mostly complex siliceous rocks shattered by volcanic 

 explosions. The more or less indefinite terms tuff, volcanic 

 breccia and agglomerate are commonly used. The essential 

 types are : 



Pyrorudytes: coarse, chiefly angular volcanic blocks and 

 bombs, loose or recemented by finer material — volcanic 

 breccias and agglomerates. 

 Pyrarenytcs: coarse tuffs where the grain is not above the 

 size of the ordinary sand grain (2 mm.). They gener- 

 ally show rude stratification and may contain organic re- 

 mains. {Vide, the buried cities and human remains in 

 the tuffs of Vesuvius and other volcanoes.) 

 Pyrolutytes: fine tuffs composed of volcanic dust and 

 ashes. Stratified, and enclose remains, as in the case of 

 pyrarenytcs. 



