EXCURSION VIII. 141 



easy. The river and tides have re-arranged the materials, 

 and rolled them into a perfect marine shingle; but doubtless 

 this was the origin of much of the debris which has filled up 

 so large a part of the bay. The shingle bank on which the 

 castle stands has very much the appearance of a terminal 

 moraine; and it is just in such a position that a glacier like 

 that supposed would for a long time throw a moraine down. 

 There are traces of a former terrace along the sides of the 

 bay, backing up against the mountains, in the position which 

 lateral moraines would occupy. When we said that the 

 shingle bank has kept its place for 2000 years, the reference 

 was to the establishment of the existing levels, not later than 

 the time of the Roman invasion. 



66. At the upper end of the gorge, where the open glen 

 begins, we are upon the junction in the river bed. Many 

 fine branching veins of granite run into the slate, narrowing 

 to threads as the distance increases, the granular structure 

 becoming at the same time more minute; in some places 

 granite bands are interstratified with the slate. The slate is 

 also penetrated by quartz veins, of which some are parallel to 

 the layers of slate, and others intersect them at various angles ; 

 in the former case, they conform to all the contortions of the 

 slate. In some of the larger veins the granite is coarse- 

 grained ; but usually a more compact or confusedly crystalline 

 strip separates these from the slate. The appearances are 

 finely exhibited in the front of the mountain, which is cut 

 steeply down, and shews the veins in section. It is this 

 circumstance which renders the junction here by far the 

 finest in the island. " The whole mountain," as Murchison 

 and Sedgwick express it, "abuts against the granite, which 

 moulds itself into the broken edges of the slate, and runs 

 into it through the gaps and fissures." Large veins are seen 

 to emanate from the granite below, and pierce the slate in a 

 slanting direction towards the summit. There are also many 

 fine veins and numerous alternations where the two rocks 

 approach; and here the usual changes are well exhibited 



