426 S. ALLPORT ON THE METAMORPHTC ROCKS 



certainly been exposed, it must be admitted that we have a sufficient 

 explanation of the absence of lavas or other superficial products, and 

 that only the more deeply seated masses of igneous rocks could 

 remain as evidences of former volcanic action. 



As regards the principal masses of granite in other parts of these 

 islands, it appears to me that the evidence is equally strong in favour 

 of their eruptive origin. Mr. Judd has shown conclusively that the 

 phenomena presented by the great granitic masses of the Grampian 

 Mountains are the exact counterpart of those observed in the lower 

 portions of the Tertiary volcanic masses of the Hebrides ; and, lastly, 

 with respect to the granites and syenites of the Lake-district, I 

 cannot but think that all the facts so clearly described by Mr. "Ward 

 in his recent valuable memoir*, are quite consistent with the vol- 

 canic origin of these masses. There is evidence to show that the 

 Cornish slates existed as metamorphic rocks (cleaved and sometimes 

 contorted) long before the intrusion of the granite. The contact 

 metamorphism produced thereby extends to a short distance only, 

 and may generally be distinguished from the other by microscopical 

 examination, although there will of course be a gradual transition 

 from the one to the other. Whenever clear indications of this con- 

 tact metamorphism occur among stratified rocks at a considerable 

 distance from granitic masses on the surface, it becomes extremely 

 probable that such masses exist at a very short distance below. This 

 is known to be frequently the case in Cornwall ; and I should be 

 strongly inclined to infer the existence of similar relations between 

 the granite and slates of Skiddaw Forest. 



Although there is an absence of direct evidence in the Lake-dis- 

 trict as to the geological period at which the granites and syenites 

 were erupted, there is apparently nothing inconsistent with the idea 

 that they may belong to the great series of volcanic outbursts 

 which occurred during the newer Palaeozoic epoch ; and it seems 

 to me highly probable that they may be approximately of the same 

 age as the granitic masses of Cornwall, Devonshire, and the High- 

 lands of Scotland. 



EXPLANATION OF PLATE XXIII. 



Fig. 1. Polished slice showing granite- vein in altered slate, collected close to 

 junction with the main mass near Mousehole. The slate has been 

 converted into a tourmaline-schist. Drawn half natural size. 



Fig. 2. Portion of thin slice from same specimen as fig. 1, showing the junction 

 of granite and slate. The clear spaces are quartz ; and in the granitic 

 part the light shade represents the felspar ; the dark, tourmaline or 

 mica. In the slate the two shades are tourmaline and mica. See 

 p. 408. Magnified 10 times. 



Fig. 3. Foliated tourmaline-schist close to the granite, Mousehole. See p. 409. 

 Magnified 2 diameters. 



Fig. 4. Small portion of fig. 3, showing the relation of the tourmaline to the 

 quartz grains. Magnified 60 times. 



Fig. 5. Spotted and foliated mica-schist containing a band of granular quartz 

 (a). See p. 409. Magnified 10 times. 



* Quart. Journ. Geol. Soc. vol. xxxii. 



