Geology and Mineralogy 639 



globe are treated in papers by A. Blytt, H. Hennessy, C. 

 Chree, and G. K. Gilbert. 



C. D. Walcott, in a paper entitled " Geologic Time, as 

 Indicated by the Sedimentary Rocks of North America " 

 (1893), investigates especially the rate of accumulation of 

 Paleozoic sediments in the Cordilleran Sea. The general 

 conclusion is reached that geologic time "can be measured 

 by tens of millions but not by single millions or hundreds of 

 millions of years." This is in harmony with C. King's paper 

 on "The Age of the Earth," in the same volume, in which 

 the theory of the mode of cooling of the earth is investigated 

 in the light of recent experiments on the latent heat of fusion, 

 specific heat, and expansion in melting of diabase. 



A. Daubree's paper on "Deep-sea Deposits" (1893) gives 

 an admirable summary of the results of the voyage of the 

 Challenger, and other recent explorations, on a subject of 

 profound interest to the geologist. 



Important contributions to the geology of particular re- 

 gions are given in T. Thoroddsen's " Volcanic Eruptions and 

 Earthquakes in Iceland within Historic Times " (1885), and in 

 A. Hague's " Geological History of the Yellowstone National 

 Park" (1892). 



A. Brezina's "Explanation of the Principles of Crystallog- 

 raphy and Crystallophysics " (1872) is a remarkably compact 

 and elegant exposition of Miller's crystallographic system and 

 of the optical characters of crystals. The theory of crystal 

 formation is illustrated by valuable papers by J. W. Judd and 

 C. D. Liveing. 



The short paper by E. Orton on the " Origin of the Rock 

 Pressure of Natural Gas in the Trenton Limestone of Ohio 

 and Indiana" (1891) is valuable for the clearness and beauty 

 of its scientific reasoning, and for the economic importance 

 of the subject which it treats. 



