SOME ANCIENT AND MODERN VOLCANIC HOCKS. 399 



of similar rocks from Vesuvius, as giveu by Prof. Phillips in his 

 work on that mountain, in order that they may be contrasted with 

 analyses of much more ancient lavas from Cumberland and Wales, 

 to be given hereafter. 



No. 1. No. 2. 



Silica 38-888 48-02 



Lime 17*698 10-18 



Alumina 14-127 20-78 



Protoxide of iron 12-698 P ™ tox - & 1 7-97 



Perox. J 



„ of manganese 0-010 



Magnesia 3-333 1-16 



Soda 10-000 3-65 



Potassa 1-190 7-12 



Water 2-063 trace. 



Sulphuric acid ] traces of 



Titanic acid L trace Chi. Sod. 



Copper j etc. 



No. 1, by Prof. Sylvester, is of the 1867-68 lava, its compact 

 part. No. 2, by "Wedding, represents an earlier current. Prof. 

 Phillips speaks of leucite thus * — " frequent in the lavas of Somma, 

 and ejected blocks ; less frequent in Yesuvian lava." It seems hard, 

 however, to conceive of its being in greater abundance than in the 

 examples of Yesuvian lava-flows given above ; but in some cases the 

 crystals may of course be larger, and therefore more visible than in 

 others. 



Summary. 



1. The Trachyte (or greys tone) of the Solfatara exhibits a well- 

 marked flow in the crystalline felspathic base. In the lavas of 

 Vesuvius and Albano the leucite seems, in part at any rate, to take 

 the place of the felspar of other lavas ; and the majority of the leucite 

 crystals seem to be somewhat imperfectly formed, as is the case with 

 the small felspar prisms of the Solfatara rock. 



2. The order of crystallization of the minerals in the examples 

 given is the following — magnetite, felspar in large or small distinct 

 crystals, augite, felspathic or leucitic solvent. 



3. Some of the crystals first formed were broken, and rendered 

 imperfect, before the viscid state of igneous fusion ceased. 



4. Even in such modern lava-flows as that of the Solfatara con- 

 siderable changes have taken place, by alteration and the replacement 

 of one mineral by another, and this very generally in successive 

 layers corresponding to the crystal- outline. 



III. Examples oe Microscopic ItocK-STPJTCixiRE among the 

 Lavas and Ashes oe Wales. 



a. Aran and Arenig Felstones. 



Prof. Ramsay has described the rock of the Arans as much resem- 



* Vesuvius, p. 292, 

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



