J. CLIFION WARD ON THE GRANITIC, GRANITOID, AND 



In selecting the specimens for analysis, I have endeavoured as 

 far as possible to take samples which should be fairly representative 

 of the rocks. 



The correspondence, in chemical composition, between the spotted 

 and mica-schist is exceedingly close, although the specimens them- 

 selves were very different in appearance ; while the analyses of these 

 rocks differ considerably from those of the chiastolite slate and the 

 granite. Indeed, chemically the chiastolite slate more nearly ap- 

 proaches the granite than do the intermediate rocks, B and C. The 

 large proportion of alumina in B and C accounts for the great 

 development, especially in B, of the little concretionary spots, in 

 which the excess of alumina would appear to be secreted. Carius 

 gives a sot of analyses of spotted schist which show a higher per- 

 centage of silica than do either B or C (see Appendix, p. 9). 



Now there can be no doubt whatever about the fact of the chias- 

 tolite slate passing into the spotted schist ; and it is somewhat 

 surprising, at hrst sight, to see such a manifest decrease in the per- 

 centage of silica and increase in that of alumina. But the analyses 

 of clay slate are very variable, and it is quite possible that another 

 sample of a similar rock from the same neighbourhood would present 

 a closer correspondence in chemical composition than does A to B, 

 more nearly resembling the Welsh slate. 



The chief difference, chemically, between the metamorphic rocks 

 A, B, C, and the granite D, is that the latter shows a very marked 

 increase in the percentage of silica and a decrease in that of alumina. 

 The question therefore arises, Is it possible to consider the granite 

 in this case the extreme term of metamorphism of these rocks? 

 Prof. Fuchs, in describing a similar series of metamorphic rocks 

 occurring in the French Pyrenees*, shows that, in the extreme 

 terms of the typical mica-schist, gneiss, and granite, the silica and 

 alkalies are increased and the alkaline earths and iron are decreased. 

 But at the same time he states that the increase in silica seems to 

 have been independent of the metamorphism, and mentions unaltered 

 clay slates traversed by white quartz veins. It seems, however, 

 very hard to make this distinction ; for the formation of quartz veins 

 implies metamorphic action produced by heated waters carrying 

 silica in solution, and this action probably effected under great 

 pressure. Indeed the whole series of changes, from the unaltered 

 clay slate to the mica-schist, is one pointing strongly to aqueous 

 action under great pressure — in other words, to a very moist heat. 

 It is quite conceivable that, under such circumstances, those rocks 

 most deeply buried would become most highly silicated ; and there- 

 fore it will not do to say, in all cases, that a rock A could not have 

 been converted into another, D, because of the much lower per- 

 centage of silica in A than D ; but, at the same time, when two 

 rocks such as our C and D are found close to one another, it seems 

 highly improbable that D has been produced by metamorphism out 

 of the surrounding rock C. 



When, therefore, all the evidence in this case is taken into 

 * LconharcTs Jahrbuch, 1870, p. 717. 



