G7 



characters of the surface products of volcanic action. This connection, 

 however, becomes less and less marked as we extend our field of view. 



It was a general opinion amongst the earlier observers that in any 

 one volcanic district the rocks of the acid group preceded those of the basic 

 group, and this is true to a certain extent. SCROPE, (1) however, has shown 

 that it is not universally true. Alternations of trachytic and basaltic 

 lavas are not uncommon. Later researches have somewhat modified the 

 early views as to the nature of volcanic products by bringing into prominence 

 the rocks of intermediate composition the andesites and porphyrites. 



The sequence of volcanic rocks in three important districts the Lipari 

 Islands/ 2 ' the Schemnitz District of Hungary,' 3 ) and the High Plateaux 

 of Utah, (4) in which the three classes of igneous rocks are represented is 

 now known in some detail. In each of these cases immense eruptions of 

 andesite were succeeded by eruptions of rhyolite and basalt. 



It would be out of place to consider the subject here referred to at 

 any length in a work devoted mainly to descriptive petrography. The cases 

 mentioned will be sufficient to show that in any one cycle of volcanic activity 

 there is a connection between age and petrographical characters. 



If now we extend our field of view, so as to take into consideration the 

 products of two or more cycles, then the connection above referred to becomes 

 much less marked ; at the same time, if we have regard only to a 

 comparatively limited area, it is frequently possible to establish differences 

 between the rocks of different periods. Compare, for example, the basic 

 igneous rocks of pre-Tertiary with those of Tertiary age in Germany, Austria 

 and Hungary. A marked difference is at once apparent. The felspathic 

 constituent of the former is always felspar proper, whereas that of the latter 

 is frequently leucite or nepheline. Again, if we limit the comparison to 

 the felspar-bearing rocks a marked difference in texture may be observed. 

 The former frequently possess the ophitic texture, whereas this is almost, if 

 not entirely, absent from the latter. This difference depending on texture, 

 however, absolutely breaks down if we extend our field of view so as to 

 include the North of Ireland, west of Scotland, the Faroe Isles and Iceland. 

 The ophitic texture is one of the most constant features in the Tertiary basalts 

 and dolerites of those districts. 



Are there, then, any differences in composition or texture, in the 

 volcanic products of different geological periods, which are constant through- 

 out the world ? It is impossible to give a perfectly definite answer to this 

 question in the present imperfect state of our knowledge ; but we may at 

 least call attention to the fact that rocks containing leucite, hauyn and melilite 

 are in all cases yet known of post-cretaceous age ; and that nepheline is 

 much more abundant in post-cretaceous than in pre-tertiary rocks. It must 

 be remembered, however, that these minerals are somewhat restricted in 



(1) The geology of the Extinct Volcanoes of Central France. 1858, p. 129. 

 Volcanoes, 2nd Edition. 1862, p. 127. 



(2) JUDD. G.M. 1875, p. 10. 



(3) Q.J.G.S., Vol. XXXII. 1876, p. 292. 



(4) BUTTON. High Plateaus of Utah, Mem. U.S. Geol. Survey Rocky Mts , 1880. 



