422 REACTION OF THE INTERIOR OF THE EARTH 



on geology, have taught us that the three great groups 

 of volcanic or eruptive rocks (trachytes, phonolites, and 

 basalts), which, regarded in great masses, are different 

 from each other in age, and most often occur very 

 widely apart, are, however, all three more recent in 

 their appearance than the plutonic granites, diorites, 

 and quartzose porphyries, and than all silurian, second- 

 ary, tertiary, and quartary (pleistocene) formations ; and 

 that they even often intersect loose beds of diluvial 

 gravel and ossiferous breccias. Rozet has made the 

 important remark, that a striking multiplicity ( 582 ) of 

 such intersections are found congregated together within 

 a small space in Auvergne; for if the great trachytic 

 mountain masses of the Cantal, Mont Dore, and Puy de 

 Dome break through granite itself, and sometimes (as 

 between Vic and Aurillac, and at the Griou de Mamon) 

 enclose large fragments of gneiss ( 583 ) and limestone, yet 

 we also see trachyte and basalt intersect as dikes, gneiss, 

 carboniferous rocks, tertiary and diluvial strata. Basalts 

 and phonolites, which are nearly allied to each other (as 

 is shown by the Bohemian Mittelgebirge and Auvergne), 

 are both later formations than the trachytes, which are 

 often intersected by dikes of basalt. ( 684 ) Phonolites are 

 older than basalts, in which they have never been found 

 forming dikes, whereas dikes of basalt often traverse 

 porphyritic schists (phonolites). In the Andes of Quito 

 I found the basalts locally separated from the prevailing 

 trachytes, occurring almost alone at the Eio Pisque and 

 in the valley of Gruaillabamba. ( 585 ) 



As in the volcanic high plain of Quito everything is 

 covered over by trachytes, trachyte conglomerates, and 

 tufa, my most earnest endeavours were bent to discover 



