484 S. AUport — Recks of Brazil Wood, Charnwood Forest. 



It will be seen, then, that this schist is neither a gneiss nor a true 

 mica- schist ; it belongs, in fact, to a group of rocks which have 

 not yet received distinctive names, although they are far from 

 uncommon. They occur round the skirts of granitic masses, 

 especially when the latter have broken through clay-slates. When 

 the prevailing constituent is mica, and they are more or less fissile 

 in one direction, they may for the present be called micaceous 

 schists, in order to distinguish them from normal mica- schists. This 

 Charnwood rock is singularly like some of the Devonian slates near 

 their junction with the Land's End granite, it is not more highly 

 altered, and the present crystalline condition of both may clearly be 

 explained by the same cause, namely, their contact with large masses 

 of intrusive granite. 



It may possibly be objected to this explanation that no such 

 amount of alteration has been produced in the sedimentary rocks of 

 the district by the intrusion of the syenite. It must not be under- 

 stood, however, that the syenite has produced no effect whatever. 

 But the two cases are not strictly analogous, and the following con- 

 siderations may probably suffice to remove the apparent difficulty. If 

 we regard the large masses of granite which occur among our 

 Paleeozoic strata as crystalline products formed at the roots of old 

 volcanos, and subsequently exposed by denudation, it is evident that 

 the surrounding rocks must have been exposed to great heat and 

 intense metamorphic action during the entire period of volcanic 

 activity ; whereas rocks lying at a distance would receive no morO 

 heat than that given out by any gradually cooling masses which 

 might be intruded among them. In the first case, the heat would 

 be far more intense and continuous ; in fact, its duration might 

 probably be measured by an entire geological period ; while in 

 the second, the heat would be far less at the commencement, 

 would be a gradually diminishing quantity, and the duration 

 of its efficient action might be reckoned by years. Sheets 

 of molten matter intruded among rocks at a distance of a very 

 few miles would everywhere meet with cold surfaces, and would 

 necessarily be cooled down at a comparatively rapid rate. It 

 has frequently been observed that granite has produced greater 

 alteration in contiguous strata than other intrusive rocks ; but it is 

 also true that the amount of change is very unequal, sometimes being 

 comparatively slight. Such facts are quite in accordance with the 

 theory here advocated. Leaving out of consideration any inequality 

 due to differences of composition in the rocks attacked, it is probable 

 that prolonged volcanic action would vary greatly in intensity at 

 different times and in various directions. It might then easily occur 

 that the molten material of granite forcing its way through strata 

 towards the close of a volcanic period would produce far less effect 

 than in the earlier stages of activity ; or the occasional transfer of 

 the volcanic source from point to point during periods of varying 

 length would also account for very considerable variations in the 

 amount of alteration produced. 



The slaty rock overlying the micaceous schist is very fine grained, 



