BOOK NOTICES. 



Chapter XVI contains descriptions of some very interesting 

 examples of contortions in bedded rocks, illustrated by numerous 

 plates. These include the contorted Carboniferous Limestone of 

 Draughton, Yorkshire, the folded Silurian beds of Wigtonshire and 

 of Aberystwith, and the curiously contorted schists or ' gnarled beds ' 

 of Anglesey. The author inclines to regard these last as Archaean, 

 as claimed by Dr. Hicks and Dr. Callaway (the citation of Professor 

 Hughes' name in this connection must be an oversight) ; and he 

 agrees with these geologists, in opposition to Sir A. Ramsay, in 

 regarding the foliation as indicative of bedding. In any case these 

 rocks afford striking instances of the various ways in which a rock- 

 mass may yield to lateral pressure. 



The next succeeding chapters deal with the various types of 

 mountain structure exhibited in the Western States of America, and 

 so ably deciphered and described by the surveyors ; these confirm the 

 authors theory of the formation of folds by 'compressive extension.' 



The author goes on to consider the connection between vulcanism 

 and mountain building. He finds the source of the volcanic ejecta- 

 menta in the intensely heated and expanding plastic rocks, which on 

 approaching sufficiently near to the surface, become fluid by relief of 

 pressure, and may be forced out as lavas. The author quotes the 

 general conclusion arrived at by Captain Dutton from his extensive 

 study of the volcanic rocks of the High Plateaux of Utah : 'Volcanic 

 phenomena are brought about by a local increase of temperature 

 within certain subterranean horizons.' 



The larger bendings of the earth's crust, involving the elevation 

 and subsidence of extensive regions, are ascribed to fluctuations of 

 temperature in portions of the deep-seated interior far below the 

 thirty-mile zone : but it is not made very clear that any adequate 

 cause exists for internal changes of temperature, other than the uniform 

 secular cooling which is usually held to account for the movements 

 in question. The author suggests chemical recombinations, and also 

 the removal of matter from the interior by discharges of lava ; but 

 changes of level are now in progress in such countries as Scandinavia, 

 where no volcanoes have existed for long periods. 



The author next shows that evidence of lateral movement is not con- 

 fined to highly plicated rocks, quoting in illustration his own observa- 

 tions on the Trias of Cheshire, etc., where approximately horizontal 

 sli(^kensides attest more or less movement in a lateral direction. 



Chapter XXIV deals with slaty cleavage and foliation. It was 

 long ago shown by Mr. Sharpe that rocks which possess the charac- 

 teristic structure of slaty cleavage have experienced a lateral com- 

 pression of their mass, in the direction perpendicular to the cleavage 



Naturalist, 



