ASSOCIATED METAMORPBTC ROCKS OF THE LAKE-DISTRICT. 29 



beneath some line of weakness in the overlying rocks, then the great 

 pressure (one of these agents) under which granitic rocks seem in- 

 variably to be formed, is likely to relieve itself in volcanic outbursts, 

 and the granite may be said to be directly the root of the volcano. 

 But if, on the other hand, these agents are at work at very great 

 depths beneath the surface, the pressure under which the granite 

 may be formed, although very intense in its degree, and far exceed- 

 ing that due to the superincumbent rocks alone, being unable to 

 open direct connexion with the surface, is spent in the work of 

 elevation and contortion of the overlying rocks. Then, if this work 

 of contortion and fracturing of the rocks above open out great lines 

 of weakness, or produce indirectly, by concomitant denudation taking 

 place at the surface on a large scale, a decrease in the thickness of 

 deposits overlying the granite, this last may then become eruptive, 

 and volcanic outbursts take place at the surface. Hence it would 

 follow, as is found to be the case, that volcanic phenomena succeed 

 or accompany the elevation of large tracts, and occur in connexion 

 with areas of depression. 



It would therefore be in the highest degree unwise to suppose that 

 every granitic mass has been the root and origin of some past series 

 of volcanic phenomena; for it may represent (1) a mass so deeply 

 formed that, notwithstanding all the elevation and contortion pro- 

 duced by the intense pressure, no point of sufficient weakness was 

 found whereby relief might be obtained and a volcanic centre be 

 established, (2) a mass which, though very deeply formed, was yet 

 able to penetrate upwards for a certain distance along some area or 

 line of weakness, though its final point of consolidation was still far 

 below the surface, with which it for ever remained unconnected by 

 any volcanic neck ; or (3) it may actually represent the very foun- 

 dation of a true volcanic neck. 



The granite of most deeply-seated formation will thus be more 

 usually surrounded by widely metamorphosed and contorted rocks, 

 of which, indeed, the granite itself may generally be considered but 

 a part, whereas the granite directly connected, at a moderate depth, 

 with volcanic phenomena will bear more clearly the marks of an 

 intrusive character. Although all volcanic phenomena may have 

 so deep-seated an origin that they are invariably connected with a 

 granitic magma, it of course does not follow that all consolidated 

 granitic masses must necessarily have been at one time connected 

 with volcanoes. If this view be true, then, in order for the pro- 

 duction of the several classes of volcanic rocks, either (A) that so- 

 called granitic magma must vary considerably in its composition 

 over different areas, and at different periods over the same area, or 

 (B) this magma must become variously changed on passing upwards 

 through and among rocks of different composition and character. 

 In the case, however, of a single vent erupting both basic and acidic 

 material at different periods, we must suppose, in the latter alter- 

 native (B), that the eruptions had their chief source at somewhat 

 different depths along the same line, where the surrounding rocks 

 by which the character of the magma might be influenced were 



