Reviews — P. Lake Sf E. H. Rastall — Text-Book of Geology. 85 



to almost endless modification, according to the structure of the 

 ground, the nature of the rocks, the effects of subsequent uplift and 

 adjustment, and the changes cansed by the denudation of strata 

 that conceal old topographical features. Attention is drawn to the 

 variations of slope that arise in the course of a stream during the 

 process of grading before it has reached the base-line of erosion. The 

 term thalweg is said to be generally employed " to express the course 

 of a stream considered in a vertical plane ". By Mr. C. S. Slichter 

 (Water-Supply Papers No. 67, U.S. Geol. Survey) the term is applied 

 to the lowest line of drainage in a valley including not merely the 

 surface flow but " the underground current, in general coincident 

 with the thalweg", and the opinion is quoted "that the subterranean 

 thalweg on the main line of underground drainage is usually nearer 

 the steeper side of the valley than is the surface stream ". 



A good picture of the Sphinx illustrates the rounding and etching 

 produced by wind-blown sand. Useful explanations are given of the 

 words dreikanter, zeugen, etc., employed by some geologists, although 

 the burdening of science with such technical terms, when ' facetted 

 pebbles' and 'tabular outliers' should be sufficient, is somewhat 

 pedantic and ill calculated to advance knowledge. 



The pictorial plates have been well chosen, and they include many 

 effective views of rock-structures, joints in igneous rock, strain-slip 

 cleavage (from the Geological Survey), vertical strata, glaciers and 

 glacial phenomena, natural arch and stacks, volcanoes and volcanic 

 phenomena (including the ' spine ' of Montagne Pelee), Tertiary basalt 

 dyke, etc. The plates all illustrate Mr. Rastall's portion of the 

 volume. 



Mr. Lake commences with a philosophic essay on the principles of 

 stratigraphy and on the methods of correlation. By means of 

 instructive diagrams he shows the theoretical distribution in space 

 and time of a species, from what would in old days have been termed 

 a 'centre of creation'; but, as the author remarks, "many species 

 had no definite beginning and no definite end, and their vertical 

 ranges accordingly are also ill-defined." Species with a wide range 

 were usually held to have a corresponding duration of time, but this 

 does not apply to all forms, aud those of zonal value " must belong to 

 a group in which evolution is pi'oceeding rapidly ". 



From the Pre -Cambrian or Archaean, in upAvard succession, the 

 author describes the main characters and subdivisions of the great 

 geological systems, and with the aid of maps points out their 

 distribution in the British Isles. The more important facts are 

 clearly laid before the reader without much detail ; thicknesses of 

 strata and economic products are not dealt with, unless incidentally. 

 The fauna and physical conditions, the characteristic fossils and zones 

 of the different formations, are the chief subjects. 



The Devonian Series of North Devon and West Somerset is 

 illustrated by Etheridge's map of 1867 — essentially a diagram — and, 

 as Mr. Lake admits, so far as our present knowledge goes, there is 

 definite palaeontological evidence that the Morte Slates are Lower 

 Devonian. 



We are glad to observe that the author has been bold enough to 



