SECRETIONS 721 



17. Secretions. Secretions are deposits formed on the walls 

 of cavities in rocks, the first layer being the outer one of the secre- 

 tion. A constant supply of material may bring about the filling 

 of the cavity, as in veins and agate geodes, while the cutting off of 

 tlie supply results in the production of hollow geodes, lined with 

 crystals. 



All secretions are secondary rock structures, and they are men- 

 tioned in this chapter merely for convenience in comparison with 

 concretions. Of the latter, incretions and excretions are secondary 

 structures, and the same is true largely if not wholly of intercre- 

 tions. Accretions are. however, extensively represented by con- 

 temporary concretions, though many kinds of true accretions are 

 also formed secondarily in strata of various kinds. 



BIBLIOGRAPHY XVII. 



1. BARRELL, JOSEPH. 1906. Relative Geological Importance of Con- 



tinental, Littoral and Marine Sedimentation, Part III. Journal of 

 Geology, Vol. XIV, pp. 524-568. 



2. BERKEY, CHARLES P. 1906. Paleogeography of Saint Peter Time. 



Geological Society of America, Bulletin, Vol. XVII, pp. 229-250. 



3. BRANNER, JOHN C. 1901. Origin of Ripple Marks. Journal of 



Geology, Vol. IX, pp. 535-536. 



4. CANDOLLE, CASIMIR DE. 1883. Rides formus a la surface du 



sable depose au fond de I'eau et autres phenomenes analogues. Archives 

 des vSciences Physiques et Naturelle, Geneve. No. 3, Tome IX, pp. 

 241-278. 



5. DALY, REGINALD A. 1900. The Calcareous Concretions of Kettle 



Point, Lambton County, Ontario. Journal of Geology, Vol. VIII, pp. 

 135-150. 



6. DARWIN, GEORGE H. 1883. On the Formation of Ripple Mark 



in Sand. Proceedings of the Royal Society of London, Vol. XXXVI pp 

 18-43. 



7. FAIRCHILD, HERMAN L. 1901. Beach Structure in the Medina 



Sandstone. American Geologist, Vol. XXVIII, pp. 9-13, pis. II-IV. 



8. FLEMING, J. A. 1902. Waves and Ripples in Water, Air and ^ther. 



London. 



9. FORCHHAMMER, G. 1841. Geognostische Studien am Meeresufer. 



Neues Jahrbuchfiir Mineralogie, etc., pp. 1-38, t. III. 

 9a. FOREL, F. A. 1883. Les Rides de Fond etudius dans le lac Leman. 

 Archives des Sciences Physiques et Naturelles, Geneve, (3.) Tome X, 

 pp. 39-72. 



10. FRAAS, O. 1872. Heuglin's geologische Untersuchungen in Ost Spitz- 



bergen. Petermanns geographische Mittheilungen, Bd. XVIII, pp. 275- 

 277. 



11, GILBERT, G. K. 1877. Report on tlio Geology of the Henry Mountains. 



United States Geographical and Geological Surveys of the Rocky Moun- 

 tain Region. 



