280 



ing gradually eastward to sea level, in the same way as it does from 

 Rory's Glen. 



Here I pause a while, and return to examine a matter I have not yet 

 touched upon. 



An evident difference takes place between the present condition of 

 the chalk to the south and to the north of Kilroot, No. 9. To the south 

 it has a steep and sudden outcrop, under a steep bank of trap, with the 

 surface of the inferior soft red marls sloping rapidly to the east ; to the 

 north, although the steep bank of trap continues, the character of an 

 outcrop is lost ; for the chalk spreads over the country for two or three 

 miles wide, and forms the surface rock in many places to the sea shore. 

 What is the cause of this outcrop of the chalk in a part of its course, 

 and not in another ? The most natural solution of the outcrop, and 

 what is first suggested by viewing it from the vicinity of Belfast, where 

 it can be seen from a distance, would be to suppose a fault along the 

 line, crooked as it is, and a downthrow to the east, in which the surface 

 would occupy all the low ground between the base of the steep slope 

 and the shore. But this is not the case : if it were, the low ground 

 would be all trap, the same as the mountains ; it is, on the contrary, 

 composed of red sandstone, and the other rocks that usually underlie 

 the chalk. We must look for the solution in some other way. 



At Balmer's Glen, JSTo. 2 in the Table, the chalk stands at an eleva- 

 tion of 230 feet. At Kilroot, No. 9, it is 490 feet. Between these two 

 stations the outcrop rises into the form of a flat arch, being from 600 to 

 700 feet high in the middle, opposite to Belfast. The elevation of this 

 part of the chalk may be in some way connected with the steep escarp- 

 ment and the present outcrop. 



The thickness of the chalk at the White Bock quarry would show that 

 the original outcrop, which might be expected to be thin, was not on 

 this line. It most probably extended a mile or two farther eastward, 

 and had a zone of bare chalk along the eastern margin of this breadth, 

 such as there is at Moira and at Ballygalley. 



I have shown that the outcrop near Belfast is not occasioned by a 

 downthrow to the east. There is no other alternative to account for it 

 but the action of denudation. 



The whin dykes, which seem more numerous along the outcrop of 

 the chalk than elsewhere, appear to have an important influence in 

 keeping it up to the elevation it had attained at the time of the protru- 

 sion of the trap. I have shown that there are five of those whin dykes in 

 one quarry at Aughnahough (Fig. 1) ; they occur from three to six 

 yards asunder. There are four or five more in a quarry at Ballymoney 

 (Big. 2), between stations Nos. 4 and 5. If they are thus seen so nu- 

 merous in quarries, where they have been exposed, it may well be sup- 

 posed that they exist along the edge of the trap, the whole way, in 

 equal or nearly equal numbers. In fact, those fissures appear quite 

 sufficient to afford space for the eruption of all the trap of the moun- 

 tains ; but the vents by which the trap was erupted were not confined 



