368 SCIENCE PROGRESS 



of organic matter in mud ; (9) pit and mound structures due 

 to small upward currents in rapidly flocculated sediments. 

 Valuable illustrations of these types of impressions are given, 

 and the conclusion is arrived at that impressions due to falling 

 rain are only a small percentage of structures usually so 

 described. Hence there is need for caution in the interpreta- 

 tion of deposition conditions based on the occurrence of so- 

 called raindrop impressions. 



From observations on present-day shores P. E. Raymond 

 {Amer. Journ. Sci. (5), 3, 1922, pp. 108-14) has accumulated good 

 evidence that the trails on sandstones and shales usually ascribed 

 to annelids are really made by gastropods and other short-bodied 

 animals. Winding irregular trails with sharp turnings must 

 have been made by short, not elongated, animals. The trails 

 of the common earthworm are nearly straight, or in curves of 

 long radius. These observations are applied to certain trails 

 described by Walcott in pre-Cambrian rocks. 



The preservation of trails requires rather unusual conditions, 

 particularly that the mud shall contain sufficient cement, such 

 as calcium carbonate, iron oxides, or hydrous siUca, to set 

 quickly on exposure. On the basis of Raymond's observations 

 S. Powers {ibid., pp. 10 1-7) describes certain marks on Penn- 

 sylvanian Sandstones in Texas as the trails of gastropods. 



Other notable papers on sedimentary rocks and structures 

 are the following : 



W. A. Johnston, " Sedimentation in the Fraser River Delta," Canada 



Geol. Surv. Mem., 125, 1921, p. 46. 

 W. A. Johnston, " The Character of the Stratification in the Recent Delta 



of Fraser River, British Columbia," Journ. Geol., 30. 1922, pp. 115-29. 

 C. K. Wentworth, " A Scale of Grade and Class Terms for Clastic Sediments," 



ihid., pp. 377-92. 

 G. D. Hubbard, " Colloids in Geologic Problems," Amer. Journ. Sci. (5), 4, 



1922, pp. 95-1 lo. 

 W. A. Tarr, " Cone-in-Cone," ihid., pp. 199-213. 

 W. A. Tarr, " Syngenetic Origin of Concretions in Shale," Bull. Amer. Geol. 



Soc, 32, 1921, pp. 373-84. 



F. A. Wilder, " Some Conclusions in Regard to the Origin of Gypsum," 



ibid., pp. 385-94. 

 A. N. WiNCHELL and E. R. Miller, " The Great Dustfall of March 19, 1920," 



Amer. Journ. Sci. (5), 3, 1922, pp. 349-64. 

 R. Thiessen, " Compilation and Composition of Bituminous Coal," Journ. 



Geol., 28, 1920, pp. 185-209. 



G. V. Wilson, " The Ayrshire Bauxitic Clay," Mem. Geol. Surv. Scotland, 

 ^■?^ 1922, p. 28. 



A. Gilligan, " Petrography of the Millstone Grit of Yorkshire," Quart. 



Journ. Geol. Soc, 75, pt. 4, 1920, pp. 251-94. 

 P. E. Raymond, " The History of Corals and the ' Limeless ' Ocean," Amer. 



Journ. Sci. (5), 2, 1921, pp. 343-7- 



