^Cs Atcounl oftheWhynn Dykes in the 



parallelism of these strata. Tliis too is the case with out* 

 own whynii dykes at Fairhead ; but oF the six dykes at 

 Bengore pronrontory, this fourth is the only one where any 

 thing like a subsiding or depression of the strata can be 

 observed. 



This dyke is so accessible, that we are enabled to exa- 

 mine its material and internal construction, from which 

 we are precluded in the former cases ; the basalt of which 

 this is composed, though contiguous to, or rather mixed 

 with, the Causeway- pillars, is very different from the Cause- 

 way basalt ; it is somewhat coarser, more granular in the 

 fracture, and though darker than the gray whynn-slone of 

 the Fairhead pillars, it resembles their colours, more than 

 the fine blue of the Causeway basalt. 



The Causeway dyke is 13 or 16 feet thick, sometimes 

 quite solid, sometimes shivery; it is entirely composed of 

 ^mall trapezoidal prisms, their sides about an inch each, 

 aiid their axes horizontal ; they are stronglv agglutinated 

 together; and when this wall is attacked by the^slcdge, it 

 sometimes breaks into fragments composed of an accumu- 

 lation of the smaller prisms, abundance of which are 

 scattered about the foot of the precipice. 



The fifth dyke is at the eastern point of the semicircular 

 bay, of which the Giant's Causeway forms the western 

 pomt; it is inaccessible, and visible only from the water; 

 it cuts vertically three or four strata of table basalt, also a 

 great stratum of red ochrcous matter, and is then lost in 

 the precipice*. 



♦ When I discoA'ered this wh^nn dyke in the year 1801. 1 was prevented 

 fron\' examining it Hccurntely hy a heavy snrf,'>?vhich deterred me from 

 venturing among the sunken rocks at the foot of the precipice ; the next 

 summer I was more fortunate, and enabled twice to reach the bottom of the 

 cliff, where the dyke inmiergcd into the water perpendicularly. 



1 traced it downwards a's it cut the horizontal strata of table basalt verti- 

 cally, aad observed each of these merging into its solid mass without any 

 the least separation of the material; each stratum, having then as it were 

 passed through the dyke, resumed its former position on the other side at 

 the same level it held before, about forty yards from the place where 

 the dyke immergcd in deep water; it arose again ten or twelve feet 

 above the surface, continuing its course due north for thirty yards, exactly 

 Hke a wait, showing the horizontal prinns of which it was constructed, 

 whose bases formed the siirface of the wall. 



The most curious part of this dyke is discovered by tracing it up the clifF, 

 whose summit it reaches a little to the eastward of its original course; here 

 it projects boldly from t!:e face of the rock like the rectangular corner of a 

 miehtv wall afbout twenty feet thick: yet this curious wall is not entirely 

 dyke, but only its west side, which, at its termiriation, shows the horizontal, 

 prisms composing it ; the east side is formed by a range of vertical pillars 

 fifty feet loirg, pari 9f ^a great columnar stratum which the dyke there cuts 

 through. 



The 



