212 SCIENCE PROGRESS 



Grabau, A. W., Stratigraphical Relationships of the Tully 

 Limestone and the Genesee Shale in Eastern North America, 

 ibid. 945-58. 



Grabau, A. W., and O'Connell, MarJorie, Were the 

 Graptolite Shales, as a Rule, Deep or Shallow Water Deposits ? 

 ibid. 959-64. 



Barrows, A. L., Geologic Significance of Fossil Rock- 

 boring Animals, ibid. 965-72. 



The nine papers listed above are the products of a sym- 

 posium held by the Geological Society of America on the 

 interpretation of sedimentary rocks. This fascinating study 

 is now attaining a great measure of popularity on both sides 

 of the Atlantic, and aims at a reading of geological history 

 informed mainly by the characteristics of the sedimentary 

 rocks that build up the geological column. The longest and 

 most important paper in the series is that by Prof. J. Barrell 

 on sedimentary rhythms and the measurement of geological 

 time. This contribution is worthy of a much more extended 

 notice than can be given here. Briefly it aims at exhibiting 

 the congruity of the evidence of the sediments, especially 

 of their rhythmic alternations, regarding the age of the earth, 

 with that afforded by the radioactive minerals. On the pre- 

 sent data Prof. Barrell regards an estimate of 1400 millions of 

 years as probable for the age of the first great invasion of the 

 Laurentian granites. The beginning of the Cambrian is esti- 

 mated at between 550 million and 700 million years. Palaeozoic 

 time is believed to have endured between 360 million and 540 

 million years ; Mesozoic time between 135 millions and 180 

 millions; and Cainozoic time from 55 millions to 65 millions. 

 Many problems in the accumulation of sediments are dealt 

 with in a novel and stimulating manner. Finally the relations 

 of organic evolution and of the source of solar energy to this 

 vast expansion of geological time are examined. 



Kindle describes interesting experiments upon the relative 

 rates and sequence of the deposition of different sediments 

 in salt and fresh water respectively. Clay, powdered chalk, 

 and sand were fed into an experimental tank in the same order 

 and amount to both media. In salt water the beds settled in 

 proper order and with sharp lines of separation between the 

 different materials. In fresh water, however, the order of 

 superposition was different, because the clay was largely held 



