THE XATURAL HISTORY OF IGXEOUS ROCKS. 21 



Iceland, etc.. regarded as survivals of the Tertiary vol- 

 canic era, are instances of this localisation ; the Triassic 

 eruptions of the southern Tirol stand in a similar relation 

 to the Hercynian series, and the eruptions of Silurian age 

 in Ireland and the Menez-Hom district of Brittany to a 

 still older series. Sporadic volcanic centres, such as those 

 of the puys of Auvergne. and multitudes of small vents, like 

 those whose craters are still preserved in the Eifel, are 

 characteristic of a late phase in volcanic evolution. In the 

 Carboniferous volcanic province of Scotland Geikie has 

 expressly distinguished a period of " puy " eruptions fol- 

 lowing that of the more important and widely-spread 

 "plateau" out-pourings. Moreover, he has identified in 

 Fifeshire and East Lothian the sites of many of the old 

 vents, now marked sometimes bv masses of coarse aQf- 

 glomerate and breccia, sometimes by a plug of rock con- 

 solidated in the throat of the old volcano. 



Yon Richthofen has reminded us that we are not 

 warranted in assuming the familiar apparatus of crater 

 and cone to be a necessary and invariable concomitant 

 of surface volcanic action, and it may be inquired whether 

 this phase is related to a distinct stage in the evolution 

 of a volcanic region. It is, of course, to be borne in mind 

 that if volcanoes of what we commonlv reo^ard as the 

 characteristic type have been formed in the earlier stages 

 of activity of a given period, they may often have been 

 obliterated subsequently or concealed beneath later accumu- 

 lations, and indeed the destruction of old volcanoes by a 

 revival of explosive action is a fact of which we have 

 direct evidence. Reviewing, however, the rather scanty 

 information at our disposal we are inclined to regard it 

 as more than a chance coincidence that, where the actual 

 structure of true volcanoes has been recognised among the 

 older formiations, it is always in connection with the later 

 maturity and gradual waning of igneous activity. The case 

 of Monzoni, described by numerous writers, is an example, 

 and the same rule holds as regards the Carboniferous and 

 Permian volcanoes of Scotland, and. still earlier, in the 

 Lower Palaeozoic volcanic districts of this countrv. In 



