EXOGEXETIC ROCKS 285 



The relative claims of the various agents active in the production 

 of a given clastic rock once determined — provided there is more 

 than one agent — it will be found that most rocks fall under one of 

 five groups, though some rocks may fall so precisely half way be- 

 tween two groups that it becomes a matter of individual opinion 

 where it should be placed. The five principal groups are : 



I. Pyroclastic rocks, or pyroclasts. 



Autoclastic rocks, or autoclasts. 

 Atmoclastic rocks, or atmoclasts. 

 Anemoclastic rocks, or anemoclasts. 

 Hydroclastic rocks, or hydroclasts. 

 Bioclastic rocks, or bioclasts. 



The interrelations of these rocks are shown in the following 

 diagram (Fig. 39), on page 286. 



Textural Groups. 



In these divisions the grain of the rock or its texture is of 

 greater importance than the composition, for, although the latter is 

 often the prime cause of selective destruction of rocks, yet this is 

 generally only on a minor scale, and the destruction of the different 

 rocks by the same agent, especially when mechanical, goes on with- 

 out much reference to the composition. On the other hand, the size 

 of the grain is generally directly proportional to the intensity of ac- 

 tion of the agent (aided of course by the composition and struc- 

 ture of the original rock), and so naturally takes a higher classi- 

 ficatory rank. These types of texture or grain of clastic rocks are 

 recognized : 



1. Rudaceous * texture, or that of the rubble rocks, in which the 

 grain is larger than that of a sand grain. Consolidated rocks 

 of this texture are Rudytes ( psephytes of many authors). 



2. Arenaceous f texture or that of sand rocks, irrespective of 

 composition, in which the size of the grain varies from being 

 barely visible to that of a grain of rice. Consolidated rocks 

 of this texture are Arcnytes (Psammytes of some authors). 



3. Lutaceousi texture or that of impalpable powder or rock 

 flour. Consolidated rocks of this type are Lutytes (i. e.. 

 mud rocks). (Pelytes in part of some authors.) 



* From rudtis, Lat. for rubble, 

 t From arena, Lat for sand. 

 I From lutum, Lat. for mud. 



