283 



manner. Dr. SORBY describes specimens from Langdale. These are 

 composed of fragments of " vesicular lava or imperfect pumice, the basis of 

 which varies from perfect glass to a completely devitrified felsite, and often 

 shows a perfect tiuidal structure." Such a rock must evidently have been 

 formed by the breaking up of a more or less glassy lava before the period 

 of final consolidation. The same author states that " in other localities the 

 ash must have contained a much more perfect pumice, and in others a 

 considerable amount of augite, subsequently altered into a green fibrous 

 mineral, which also entirely or partially h'lls what were originally cavities in 

 the rock Sometimes even in the same microscopical section we can trace 

 a gradual passage from ash composed of particles large enough to be easily 

 recognized to what appears to have been an extremely fine-grained volcanic 

 dust, associated with fragments of true pumice." Many of the fragmental 

 volcanic rocks in question have been converted into slates and in these 

 cases the constituent particles have been so deformed that it is often 

 impossible to determine their original characters. 



In many parts of the Lake District and especially round the granite 

 masses of Eskdale, Wastdale and Ennerdale there occur compact felstone- 

 like rocks often more or less porphyritic about the origin of which it is 

 difficult to be certain. Mr. WARD was led in the course of his mapping 

 to regard these as highly altered ash-beds, but for details as to the 

 evidence on this point the reader is referred to the original communications. 

 The following analyses taken from Mr. WARD'S papers will give some 

 idea of the chemical composition of the volcanic rocks of the Borrowdale 

 series. 



I. II. . III. iv. 



Si0 2 60-718 59-511 59-151 68-421 



Al,0 14-894 17-460 19-212 15-855 



Fe 2 3 1-405 1-271 -879 -172 



FeO 6-426 4-926 5-192 2-855 



CaO 6-048 5-376 5-208 2-016 



MgO 1-909 1-801 1-909 -792 



K 2 2-354 3-705 2-933 3-338 



Na a O 2-843 3-093 4-217 5-627 



FeS '395 -604 -360 



P 2 -281 -115 -439 -204 



SO, -103 -086 tr. 



C0 2 1-660 1-569 tr. tr. 



Loss -964 -483 -500 -720 



100-000 100-000 100-000 100-000 



I. Pyroxene -porphyrite (Felsi-dolerite uf WARD) Browii Knotts, near Keswick. 

 II. Pyroxene- porphyrite Iron Crag. 



III. Probably a lava ; close to granite of Lingmell Beck, near Wastdale Head. 



IV. Coarse ash from Slight Side, about a mile from the granite (Sheet 98, S.W.) 



It will be observed that the ashes are, so far as they have been 

 analysed, more acid than the lavas. 



