638 Scientific Proceedings, Royal Dublin Society. 



Course of the Eastern Limiting Anticline. 



The tracing of the eastern anticline southward from the Torr Head area is 

 a much more difficult problem, as the older rocks do not show on this side, 

 and we have only the much less definitely marked posthumous deformation 

 of the lavas and Mesozoic rocks to guide us. One is tempted at first to regard 

 this axis as parallel to the coast of Antrim, but on closer examination of the 

 evidence this does not seem to be the ease. It is a remarkable fact regarding 

 the basalt of Antrim that the chalk is, throughout three-quarters of the 

 periphery at least, continuously preserved beneath it. This chalk is, 

 geologically speaking, a mere film, being seldom more than 100 feet in thick- 

 ness, and only at one point reaching 200 feet. In pre-basaltic times this 

 " film" was subjected Lo subrenal denudation,. as is proved by its piped and 

 Hint-covered surface beneath the basalt. Nevertheless, it has not been to 

 any extent dissected by river valleys, as the basalt does not trench down into 

 the softer. rocks beneath. This fact seems to me to indicate that it was laid 

 down horizontally, and maintained an approximately horizontal or at the most 

 a gently inclined position till the lavas were spread over it. If this assumption 

 be correct, then the contours of the chalk will give an indication of the lines 

 followed by the post-Cretaceous deformation. At the two northern angles of 

 the basalt plateau it reaches high altitudes (750 feet to 1600 feet), where, as 

 we have shown above, the transverse anticlines intersect the elevated axis 

 of the Highland border. At the south-west corner it is not preserved. At 

 the south-east comer it reaches altitudes of 650 to 720 ft. over a belt about 

 six miles wide around Divis Mountain, due west of Belfast. This is 

 where I consider the easterly transverse anticline intersects the escarp- 

 ment over the Lagan Valley, and that escarpment lies along the margin 

 of the southern uplands as defined by the projected edge of the trough- 

 valley. 



As regards the course of the eastern anticline between the Torr Head 

 schist area and Divis Mountain the evidence is not quite so definite. The 

 high levels of the chalk-outcrop along the coast as far south as Glenarm 

 seem to indicate that the axis of elevation follows the coast for tins distance. 

 Here, however, it appears to take a slightly more southerly course, passing- 

 inland, and leaving to the east areas where the faulting and dip bring the 

 chalk to progressively lower levels, and by the time Divis Mountain is 

 reached its course appears to run almost due south. We have no means of 

 tracing it further across the valley of the Lagan into the Silurian area of 

 Co. Down. 



