80 Prof. Liveing, On the metamorphism [Oct. 29, 



ture to be due to exposure. The veins are mostly felspatliic, but 

 in some cases contain various minerals, iron pyrites, galena, calcite 

 and epidote. With one exception, which I shall mention hereafter, 

 they are all small and plainly not intrusive, but connected with 

 the more highly crystalline character of the rock in the places 

 where they occur. This highly crystalline character shews itself 

 not only in the obliteration of all stratification, but in the more 

 complete separation of the minerals of which the rock is composed. 

 The ci'ystals become in many places much larger ; a gradual sepa- 

 ration of hornblende from felspar has gone on, each crystal seeming 

 to collect material of its own kind, until a very coarse mixture is 

 produced. This is a kind of result which we know to be produced 

 artificially in mixtures of chemicals by slow partial solution and 

 recrystallization, or partial decomposition and recomposition, under 

 variations of temperature. In such processes the smaller crystals 

 present the larger surfaces in proportion to their weight, and so 

 become dissolved or decomposed in the greater proportion, while 

 the crystallogenic law which causes the deposition of crystallizable 

 material on crystals of its own kind determines the growth of 

 larger and larger crystals, and thereby the more complete sepa- 

 ration of the several mineral species. These veiny places are evi- 

 dently places where the joints have been more open, and have 

 allowed more free passage for air, water, steam and gases, agents 

 which may have produced partial decomposition. Moreover it may 

 be observed that some of the most highly crystalline parts are 

 found near the junction of the north and south divisions of the 

 island, as in Well Road quarry and other quarries near St John's 

 Church in the town of St Peter Port, and at Cobo, at a very short 

 distance from the gneiss, while the parts of the syenite which shew 

 stratification are farther north and lie deeper, as at the very 

 bottom of the deepest quarries near Yale Mill which are wrought 

 below the sea level, so that the unstratified part lies between two 

 stratified portions. There seems to have been an elevation, with 

 its centre somewhat to the east of Guernsey, producing an anti- 

 clinal axis with a generally east and west direction somewhere 

 about the line from the north of St Peter Port to Cobo, so that it 

 is just in this part that the joints would be most opened by the 

 process of elevation. It can hardly be supposed that these middle 

 portions can have undergone fusion by heat, and I doubt whether 

 the crystalline structure of granite is ever due to cooling from 

 fusion in the sense generally understood, but is rather due to the 

 metamorphic process before mentioned which will go on at com- 

 paratively low temperatures, but which necessitates, not a uni- 

 formly tlecreasing temperature, but one of continued variations. 

 Variations of temperature will not in general be so frequent at 

 a great depth below the surface as at moderate depths, so that 



