igz Dr. Richardson's Letter on the basaltic Surface 
rock is similar to the contiguous consolidated masses ; but on 
the east side a singular whyn dyke is joined to it, composed, 
(as they often are,) of several walls agglutinated together, with 
wall-like fragments of other parts of the dyke emerging at 
their base ; the solid mass of dyke is seen cutting down the 
precipice to the southward at 150 yards distance. 
Depressions of the Strata. 
Soon after we have passed the last of our whyn dykes at 
Port Spagna, ( a name derived from a vessel belonging to the 
Spanish Armada having been driven ashore in that Creek), 
we discover a new and curious circumstance, viz. that the 
western half of the promontory has sunk or subsided be- 
tween thirty and forty feet, without the slightest concussion 
or derangement of the parallelism of the strata. 
Two other depressions appear as we proceed onwards, 
one at Portmoon , and the other at the angle where the pro- 
montory begins to project from the rectilineal coast ; these 
however are far less considerable in thickness than the pre- 
ceding, neither of them exceeding five feet. 
Such depressions occur at the collieries near Ballycastle , 
and generally on one side of a whyn dyke. We have also at 
Seaport, two miles west from the Giant’s Causeway, a dyke, 
oblique and undulating, with a depression of the strata of about 
four feet on one side ; but on Bengore promontory our dykes 
are unaccompanied by depressions of the strata, and where we 
havedepressions, we do not find a trace of a dyke. 
The portions of this extensive fa£ade, which I have selected 
for explanatory views, are Portmoon, in or near which most 
of the strata emerge, and Pleskin, where the strata culminate, 
