RECENT ADVANCES IN SCIENCE 213 



in suspension until all the sand had been deposited. Further- 

 more, the different classes of sediment were not sharply differ- 

 entiated, but graded one into the other. 



Prof. Grabau and Miss M. O'Connell, contrary to the gener- 

 ally-received opinion, have come to the conclusion that the 

 graptolite shales of Southern Scotland and Scandinavia were 

 deposited in very shallow water. This view is based on the 

 remarkable transition in Moffatdale from fine, thin, black shales 

 to thick, coarse, barren grits and conglomerates with marks 

 indicative of continental deposition by rivers. This transition 

 was mapped for each graptolite horizon ; and it is thereby 

 shown that Moffatdale lay in a small bay or lagoon, and could 

 not have been in the open sea. The graptolite shales are re- 

 garded as mud deposits on the flood-plain and in the lagoons 

 of a large delta, or series of deltas, where periodic high tides 

 washed in the planktonic graptolites and stranded them on 

 the flats. Occasionally they were swept into the regions of 

 deposition of coarser sediments, and are hence now found in 

 arenaceous sediments associated with worm-tracks and euryp- 

 terid fragments. 



Blackwelder, E., The Study of the Sediments as an Aid 

 to the Earth Historian, Proc. Nat. Acad. Sci. 191 8, 4, 163-7. 



In this country there has recently arisen considerable discus- 

 sion as to the origin of flint (see Science Progress, July 191 8, 

 39). In America the very similar substance chert has aroused 

 a great deal of attention. W. A. Tarr (Amer. Journ. Sci. 191 7, 

 44, 409-52) has introduced a new theory of primary colloidal 

 deposition of chert to explain the occurrence of this substance 

 in the Burlington Limestone of Missouri. Other workers, 

 however, are not inclined to accept this view. R. S. Dean 

 shows by experiment that silica hydrosols are precipitated by 

 calcium carbonate in the presence of carbon dioxide ; while, 

 in the absence of that gas, the carbonate acts as a stabiliser. 

 To this reaction he attributes the formation of chert in the 

 Mississippian and Cambro-Ordovician limestones of Missouri. 

 The groundwater circulating in these limestones is known to 

 contain a notable quantity of silica derived from adjacent 

 shales. That the chert nodules are secondary and have been 

 formed under pressure is shown by the bowing-up of the 

 intercalated shale beds in which they are mainly found (Amer. 

 Journ. Sci. 191 8, 45, 411-5). F. M. van Tuyl (ibid. 449-56) 



