CHAPTER XVII. 



SUMMARY OF ORIGINAL FEATURES OF CLASTIC ROCKS. 



We may now summarize the various structural features of clas- 

 tic rocks which were formed at the time these rocks were deposited 

 and which therefore serve as guides in the determination of the 

 mode of origin of the rocks possessing them. In dealing with each 

 feature separately it will be possible to indicate the extent to which 

 it is characteristic of one or the other of the types of clastic rocks 

 so far discussed. 



We may treat these characters under the following headings : 



1. Stratification. 



2. Cross-bedding. 



3. Beach cusps. 



4. Wave marks. 



5. Rill marks. 



6. Mud cracks (sun cracks or desiccation fissui'es). 



7. Clay galls. 



8. Clay boulders and pebbles. 



9. Rain prints. 



10. Ripple marks. 



11. Impressions made by animals in transit. 



12. Application of these structures in determining position of 



strata. 



13. Rounding and sorting of sand grains, and wearing of pebbles. 



14. Characteristics of inclusions in sand grains. 



15. Organic remains. 



16. Concretions (partly secondary). 



17. Secretions (secondary). 



Nearly all of these structural characters have been generally con- 

 sidered as preeminently if not exclusively characteristic of marine 

 or lacustrine hydroclastics. From the foregoing discussion, how- 

 ever, it will appear that many of them are far from being the ex- 

 clusive features of these types of deposits. In fact, it may be said 



6q6 



