SOME ANCIENT AND MODERN VOLCANIC ROCKS. 415 



Tertiary (Cotta) ; and, as I have endeavoured to show in the course 

 of this paper, ancient lavas having at any rate a basaltic likeness 

 occur low down in the Lower Silurian Series. And if this be the 

 case, how can we refuse to believe that trachytic lava-flows likewise 

 occurred, similar in all essential respects to those of more modern 

 times? We are certainly here met by a difficulty. Some modern 

 lava-flows, such as those of Vesuvius and the older ones of the Alban 

 Mount, are characteristically leucitic. Cotta observes, "it certainly 

 is somewhat remarkable that hitherto no ancient leucite rock has been 

 found," and the occurrence of leucite changed into orthoclase on 

 the ridge of the Erzgebirge "suggests the question whether the 

 felspar of many older rocks may not originally have been leucite whose 

 form has become indistinct or entirely altered, so as to be no longer 

 recognized " *. Future investigations may tend to confirm this sug- 

 gestion ; but at any rate Wales, with its highly siliceous class of 

 lavas, is not likely to furnish the examples. 



We have seen that it is probable that the metamorphism of the 

 volcanic rocks in the Lake-district took place during Old Bed 

 times, and most likely during the earlier part of that period; and 

 it is certainly a fact not a little suggestive, that as in the Lake-dis- 

 trict, so in North Wales, there is very little Old Bed, excepting 

 upper beds immediately beneath and conformable to the Carbonife- 

 rous Limestone. Was not the same work of upheaval and denuda- 

 tion of the Silurian rocks as a whole going on in the two districts 

 at the same time ? and if so, may not the period during which the 

 extreme metamorphism of the volcanic rocks was taking place 

 have been the same in both districts ? viz. when they Were buried be- 

 neath the maximum thickness of Upper Silurian rocks, and when 

 the slow upheaval of the ancient Upper Silurian sea-bed first com- 

 menced, or, in other words, during the earlier part of the Old Bed 

 continental period (Bamsay), in the course of which volcanic 

 activity prevailed in Scotland f, Ireland, and South-western England 

 (Middle and Upper Devonian). 



f. Classification of Cumberland Lavas. — In making a general 

 comparison between the Lake-district lavas and some of those of 

 Wales and Italy, the following points may be noted : — 



1. The order in which the minerals have crystallized out is the 

 same in the examples given, both from Italy and Cumberland — this 

 order being, first the magnetite, next the felspar and augite, and 

 lastly the small acicular felspar crystals and the felspathic or leu- 

 citic base. . r . 



2. In many of the Cumberland lavas there has been a distinct 

 flow of the smaller crystals round the larger since the latter were 

 formed, as in the case of the trachyte from the Solfatara. 



3. In the Cumberland lavas the felspar in the base seems to occupy 

 the same position as the leucite among Yesuvian lavas ; compare 

 figures 5 and 6. 



* ' Kocks Classified and Described,' English translation, p. 143. 

 t See Paper by Mr. Judd " On the Ancient Volcanoes of the Highlands," &c. 

 Quart. Journ. Geol. Soc. vol. xxx. p. 220. 



Q. J. G. S. No. 123. 2 f 



