GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 531 



vein. When the two are present in the same vein the 

 aplite always occurs in the middle of the vein with the 

 pegmatite on either side. There are, of course, many 

 small veins containing aplite only, and a few containing 

 pegmatite only ; but where the two are together there is 

 always pegmatite on either side of the aplite, though there 

 may be very much more on one side than the other. The 

 pegmatite not infrequently appears to shade off gradually 

 into the main rock, and it is never separated from it by a 

 quite sharply defined boundary, but its junction with the 

 aplite it encloses is generally quite sharp. 



The relations of the pegmatite to the aplite, and of the 

 different varieties of these rocks to one another, are never 

 suggestive of the intrusion of a consolidated rock by a 

 wholly or partly liquid magma. There are no broken frag- 

 ments of one rock in another, there is never the slightest 

 trace of a rapidly closed selvage, and there is no fluxion 

 structure. The bounding surfaces of these rocks are always 

 rounded, not angular, though where patches are of lentic- 

 ular shape they may be drawn off to points at either end. 

 No better simile could be found for the relations of these 

 various rocks than the intermixing of the different colours 

 in the plainer kinds of " marbled " paper used by book- 

 binders. There are the same flowing curves, sharp points 

 and streaks, and the same sharp edges to the varieties of 

 rock as to the different colours. 



It is impossible in words to convey a correct idea of the 

 relations of the kinds of rock in the segregation veins, and 

 we have therefore reproduced here sketches and photo- 

 graphs of such parts of the veins, easily accessible in the 

 lower part of the Reservoir Quarry, as seemed to exhibit 

 specially well in a small area structures which are common 

 throughout the great veins and occur also in others. In 

 Plate XXXVII is recorded part of the upper great segre- 



