302 MESSRS. A. HARKER AND J. E. MARIJ ON 



the same direction. Another is coarsely nodular, the spheroidal 

 nodules varying from an inch to a foot in diameter. Such nodular 

 rhyolites are well known in other districts, and have been discussed 

 bj' one of us in the case of the Ordovician lavas of Caernarvonshire *. 

 The alterations there described in what appear to have been giant 

 spherulites, and in particular their partial and total replacement by 

 crypt ocrystalline silica or quartz, are exhibited on a magnificent 

 scale on Great Yarlside and at other localities in our district. The 

 peculiarity is not confined to true lava-fiows ; for an apparently 

 intrusive rock in Blca lieck plantation, near Shap Wells, contains 

 good silicified spheroids. 



The rhyolites are never notably porphyritic, resembling in this 

 and other respects the corresponding rocks in North Wales. Indeed 

 the microscope shows that much of the material of the rocks was 

 but very imperfectly individualized into felspar and quartz, pre- 

 senting rather the features which are referred by many English 

 13etrologists to devitrification. Vesicles are found in these rocks only 

 rarely, and they are usually of microscopic size. 



An idea of the chemical composition of the rhyolites may be 

 gathered Irom Mr. Garwood's analyses given below ; the figures for 

 two Caernarvonshire rocks are quoted for comparison. It will 

 also be seen from columns I. and II. that, whatever metamorpliism 

 has operated in the rhyolite near the granite-contact, it has not 

 materially affected the bulk-analysis of the rock. 



The specific gravity of the specimen (I.) analysed is 2-60>), which 

 agrees exactly with that of similar rocks from North Wales. One 

 of the most metamorphosed rhyolites from Wasdale Head gave 2'623, 

 showing no great difference. 



SiO., 







I. 



75-95 

 13-77 



3-48 



not estim. 



trace. 



0-25 



6-55 



II. 



7fi-95 



15-50 



2 to 



not estim. 



l-*6o 



4-50 



III. 



74-88 



12-00 



3-50 



0-20 



1-28 



0-34 



r 2-49 



\ 4-77 



I 1-20 



IV. 



77-5 



ALO., 







9-7 



Fe.,0, 







61 



FeO 





... 



not estira. 



MjrO 







CiiO 









Na,0 

 K,U 

 (Ignition) ^ 



\l 



O 



c 

 u 



0-3 

 5-8 

 0-4 



100-00 10000 100-66 99-8 



I. Spherulitic Ehyolite, Stockdale ; anal. E. J. Garwood. 

 II. Nodular Rhyolite, close to granite, near Wasdale Head Farm ; anal. 

 E. J. Garwood. 



III. Rhyolite, Pitt's Head, 2^ miles S.W. of Snowdon ; anal, J. Hughes, 



Trans. Roy. Ir. Acad. vol. xxiii. (1859) p. 615. 



IV. Rhyolite, Cwm-silyn, above Nantlle Valley ; anal. E. Hamilton Acton and 



J. T. Hewitt ; ' Bala Vole. Ser. of CaerA.' (1889) p. 13. 



A very characteristic rock is that which forms the lower part of 

 the Coniston Limestone rhyolite at Stockdale and Long Sleddale. 

 It is of the laminated, fissile variety, the overlying rock being 



* ' The Bala Volcanic Series of Caernarvonshire ' (1889), chap, iii. 



