ISO 
Mr. Weaver on the 
into which the larger distinct columns of basalt, on the north coast 
of Ireland, are sometimes seen to be divided in their upper ex- 
tremity, as in the western bank below the bridge at Bushmills. 
But in Arklow rock, they form the subdivisions of a stratum. 
Toward the southern end of the hill, adjoining the coast, the rock 
is trap conglomerate, consisting of angular fragments of the com- 
pounds already noticed, embodied in a basis of greyish felspar. 
The upper and southern parts of Arklow rock are composed of 
distinct strata from l\ to two feet thick, slightly inclined to the 
south-west. These are intersected by cross parallel joints ranging 
north-west and south-east, inclining to the south-west, but nearly 
vertical ; and hence each stratum becomes subdivided into rhom- 
boidal columnar masses. This arrangement is very plainly dis- 
closed in the great quarry on the south-western side. 
The disposition of these strata is different from what we have 
hitherto observed in the clay slate tract. The greater and lesser 
rocks of Arklow, are immediately connected with each other, 
forming a higher and lower sub-conical ridge running parallel to 
the coast. I conceive this mass of trap to be enveloped in clay 
slate, which exists to the west, and re-appears to the north and the 
south at the distance of about two miles, but to possess a structure 
independent of the latter rock : and this independence of structure 
seems to be one of the characteristics of trap rock, for it may be 
remarked also in that which occurs interstratified with floetz rocks. 
In the present case, it probably constitutes a great elliptical or len- 
ticular mass, impressing its own form on the direction of the con- 
tiguous slaty strata. A similar disposition seems not uncommon in 
those tracts, in which mountain masses of quartz rock are im- 
bedded in clay slate. 
