128 NATURAL SCIENCE. Feb., 



all brain-work ; he has had to read and toil much to put these things 

 before them ; they, for their part, must take some pains to understand 

 him. As he truly says, " A popular superficiality is of service to no 

 one, and by it our science is only injured." 



The work is divided into fourteen chapters, of which the first 

 three deal with such general questions as the interior of the earth and 

 the origin of igneous rocks, the upheaval and erosion of the land, and 

 the biological, physical, and astronomical methods of estimating the 

 duration of geological time. Then follow ten chapters which describe 

 the gradual evolution of the world from Cambrian times to the close 

 of the Glacial Period, each chapter dealing with a geological system, 

 describing its living beings and the distribution of its lands and seas 

 so far as knowledge permits. Either the first appearance or the acme 

 of any important forms of life introduces a fuller explanatory descrip- 

 tion of the group to which they belong. The final chapter sums up 

 the general results from the point of view of changes of climate and 

 geography and the evolution of organic life. 



As an example of Dr. Koken's method, let us see how he treats 

 the Silurian System. 



This term is used in its older and broader sense for all strata 

 between Cambrian and Devonian. For Europe, the Dictyonema 

 Shales are regarded as passage-beds from Cambrian to Silurian, but 

 it is pointed out that the precise boundary might well be drawn at 

 many another level. Broadly speaking, the beginning of the Silurian 

 is marked by increase of trilobites, and the sudden appearance of 

 thick-shelled cephalopods and gastropods. A corresponding litho- 

 logical change is the development of limestones instead of the sand- 

 stones and clays of the Cambrian. This again was connected with a 

 general sinking of the land and extension of the sea in the most 

 characteristic Silurian areas. The enormous thickness ascribed to 

 the passage-beds in N. America gives Dr. Koken " the impression 

 that the American geologists are not yet masters of the faunistic and 

 stratigraphic difficulties." Similar passage-beds are found in China, 

 in Australia, and perhaps also in Argentina. 



This account is appropriately followed by a short description of 

 Dictyonema, and a comparison of it with other graptolites. The 

 trilobites, already described in the Cambrian chapter, have their 

 chief forms alluded to. An interesting attempt to reconstruct the 

 physical geography of the period leads on to an account of the 

 typical Silurian and its distribution. 



The lower beds of the Silurian closely follow those of the Upper 

 Cambrian in their distribution, but in many places overstep the 

 limits of the Cambrian sea. In Upper Silurian times the mainlands 

 were again more elevated, but occasional transgressions of the sea in 

 areas where Lower Silurian is unknown show that there were inde- 

 pendent movements of the earth's crust. The close of the system is 

 here and there marked by beds which could only have been deposited 

 in parts of the sea that were passing away, partly brackish, partly 

 very salt from evaporation. While communication between the 

 northern parts of Europe and America remained free, a very distinct 

 marine province ran from Belgium over France and Germany to 

 Bohemia and the Eastern Alps, and also included the Silurian of 

 Spain and Sardinia and such sporadic portions as are known in the 

 Mediterranean. In this peculiar basin the fauna was totally discon- 

 nected from that of the preceding Cambrian sea. It is possible that 

 this Silurian Mediterranean extended eastwards into Indian and 

 possibly Chinese waters. 



