SOME ANCIENT AND MODERN VOLCANIC ROOKS. 419 



3. The Lower Silurian lavas of Wales, so far as yet examined, all 

 belong to the felstone (= modern trachytic) group. 



4. The Lower Silurian lavas of Cumberland, of about the same 

 age as those of Wales, belong to the basaltic group, or stand some- 

 where midway between it and that of the felstones. 



5. In both Wales and Cumberland, felspathic ashes have been 

 metamorphosed into very felstone-like rocks. 



6. Neither the careful inspection of hand-specimens, nor the 

 microscopic examination of thin slices, would in all cases enable 

 truthful results to be arrived at in discriminating between trap and 

 altered ash-rocks ; but these methods, and that of chemical analysis, 

 must be accompanied oftentimes by a laborious and detailed survey 

 of the rocks in the open country, the various beds being traced out 

 one by one, and their weathered surfaces particularly noticed. 



7. In the present case, the microscopic examination of the Cum- 

 berland rocks, especially when accompanied by a comparison with 

 those of Wales, tends most decidedly to confirm the mapping of pre- 

 vious years. 



/Supplementary Note. 



Since the above paper was read I have examined minutely some 

 of the free quartz in the Snowdon felstone. Within the quartz space 

 seen in fig. 19 are many liquid- cavities ; some of these are purposely 

 magnified three times the rest of the figure to make them visi- 

 ble. The majority are about — ^ inch in diameter, and contain 

 vacuities moving freely in the liquid. The relative size of these 

 vacuities to the liquid-cavities is from '250 to -300, a value similar 

 to that obtained by Mr. Sorby in the case of quartz in the trachyte 

 of Ponza. I think, indeed, that there is every reason to believe that 

 the highly silicated Welsh felstones correspond to the modern quartz 

 trachytes. — J". C. Ward. 



EXPLANATION OF THE PLATES. 



Plate XVII. 



Italian. 

 Fig. 1. Trachyte from the Solfatara, near Naples, X 40. 



2. Trachyte from the Solfatara, near Naples. Viewed with polarized light, 



X15. 



3. Leucitic basalt, A r esuvius, lava of 1631, Torre dell' Annimziata, X 15. 



4. Leucitic basalt, Vesuvius, lava of 1794, Torre del Greco, X 6. 



5. The same, more highly magnified. The upper piece of augite is the south- 



western corner of the upper crystalline group in fig. 4, X 45. 



Cumberland. 



6. Porphyritic dolerite, Eycott Hill, Cumberland, X 25. 



7. Basalt, Brown Knotts, near Keswick, X 30. 



8. Trap, Latterbarrow, Wastwater. Viewed with polarized light, X 15. 



9. Altered ash with broken crystals, Bleaberry Fell, near Keswick, X 60. 



10. Bedded ash, Steel Fell, near Wythburn. Viewed with polarized light, 



Xl5. 



11. Altered streaky ash, Base Brown, Borrowdale,xl5. 



