ASSOCIATED METAMORPHIC ROCKS OP THE LAKE-DISTRICT. 599 



I think therefore, we may rather conclude that the composition of 

 the contemporaneous traps is not essentially changed by their meta- 

 morphism, and that the associated ash is either more highly silicated 

 from the first than the lava-flows, or, what seems less likely perhaps, 

 an excess of silica has been introduced by metamorphism. At any 

 rate, as the ash-deposits form the bulk of the volcanic rocks, and 

 the analyses of both fine and coarse altered ash so well agree with 

 each other and with that of the granite, there is no chemical reason 

 why metamorphism should not convert such rocks into granite, the 

 occasional lava-beds forming a very slight hindrance to so wide an 

 action. 



It would require a much more extended series of analyses to 

 establish the metamorphism from the chemical evidence alone ; but 

 this, taken in connexion with the petrological and microscopic 

 passage, is of considerable interest and importance. 



3. Order of chemical changes. — If, however, the analyses of altered 

 ash-rocks here given may be trusted as presenting a fair idea of 

 their average composition, it is clear that no addition of chemical con- 

 stituents is required to convert them into granite, with the exception 

 perhaps of a small percentage of extra silica. Now Mr. Sorby has 

 shown (Brit. Assoc. Report, 1857, p. 92) that the materials of both 

 quartz and mica may be derived from felspar which has lost a part 

 of its alkaline bases, and that such a metamorphism must be the 

 result in great part of highly heated water disseminated through 

 the rock. We know that when the Eskdale granite was formed 

 great quantities of highly heated water were present ; for the liquid 

 remains sealed in millions of minute cavities, and the size of the 

 vacuous bubble in each cavity points to the temperature and pressure 

 at and under which the liquid was enclosed ; may we not, therefore, 

 conclude that when these volcanic deposits were buried beneath 

 overlying rocks to depths of from 15,000 to more than 20,000 feet, 

 they were subject to the metamorphic influence of highly heated 

 water, and that the various stages of metamorphism were something 

 like the following : — 1st, the felspathic material (being decomposed 

 somewhat, and robbed of a part of its alkaline bases) was partly 

 converted into chlorite, some of the excess of silica and alumina 

 going perhaps towards the formation of garnets, so plentiful in the 

 ash-rocks at that stage of alteration ; 2nd, this chlorite, which at 

 the first may be regarded as a hydrous mica, became converted in 

 great part into true mica, the magnesian mica appearing first and the 

 potash-mica following, as more alumina was set free by the con- 

 tinued separation of silica ; 3rd, all the constituents became more or 

 less crystalline, the mica in the manner indicated ; the felspar, 

 formed from the reconstituted felspathic material existing in the 

 rock at the outset, was united with the quartz in one stage of gra- 

 nitic formation, so as to give rise to a quartz -felsite base ; but the 

 two minerals finally separated, the felspar crystallizing first and the 

 quartz solidifying interstitially. 



4. On the Phosphoric Acid present in the Volcanic Series. — In a 

 paper recently brought before this Society by Mr. Hicks, " On the 



