i6 SCIENCE PROGRESS. 



from the South of England and Brittany through Central 

 Europe to Saxony and Silesia. To the latest (Tertiary) 

 epoch of disturbance belongs the great Alpine system of 

 mountain-chains, with its extensions into Asia and Northern 

 Africa. The author makes it clear that each of these great 

 systems of crust-movement has its own suite of volcanic, 

 intrusive, and plutonic rocks, developed within the limits of 

 the disturbed region, and in a general sense contemporaneous 

 with the folding. 



That this contemporaneity of igneous activity with dis- 

 turbance of the solid earth-crust is to be understood in a 

 general sense will be easily apparent. It is certain that 

 movements of the kind considered proceed with extreme 

 slowness, being prolonged, interrupted, and resumed during 

 long periods of time ; and many observations go to prove 

 that they are not simultaneously felt throughout their extent, 

 but are gradually propagated in wave-like fashion through 

 the crust of the earth. There is no difficulty in believing 

 that the disturbances initiated in Eocene and Miocene times 

 have in some regions not yet wholly subsided. Again the 

 geological records of superficial volcanic action prove it in 

 many districts to have been of long duration, with intervals 

 of quiescence interrupted by renewed activity, and finally 

 gradual extinction ; while there is good reason for supposing 

 that the intrusion and complete consolidation of, say, a large 

 body of granite implies a lapse of time not always to be in- 

 cluded within the limits of one geological period. Regard- 

 ing then the four, or more, distinct times of folding, not as 

 decisive epochs, but rather as prolonged ages, having, 

 however, a more or less defined era of maximum energy, 

 and allowing a similar latitude as regards the associated 

 manifestations of igneous activity, we may expect to find 

 that in some cases the eruptions connected with one system 

 of crust-movements overlap in time those belonging to 

 another system, and that perhaps in districts not very 

 widely separated. In such cases the true relations may 

 appear only on a comprehensive view, embracing, for 

 instance, the whole of the European area. Thus in the 

 Midland Valley of Scotland igneous activity connected 



