496 Mr. Hen slow’s Supplementary Observations to 
the character of an amygdaloid, the basis of which varies considerably 
in colour, assuming every intermediate shade between light grey 
and the deepest green. This portion appears to traverse the tuff 
like a broad irregular dyke, rising at intervals in abrupt masses, the 
highest of which, denominated the Stack, is insulated, except at 
low water. It is in several places split by numerous vertical 
fissures. 
Fragments of the amygdaloid occur entangled in the tufaceous 
tract, as well as large nodules of the dark limestone, and even 
portions of its strata to a considerable extent, more particularly 
towards the south, where they have caused the appearance of an 
alternation; but I could observe no instance where the limestone 
might not be traced to an end, and be observed to feather out, 
with the evident character of an isolated mass. On examination 
we find that the embedded nodules, which occur so plentifully in 
the tuff, have been penetrated by it, and that it appears in little 
enclosed patches within the whole extent of their surface. Small 
cavities like air blebs also occur. These nodules do not appear to 
have lost their original colour or texture. Where the amygdaloid 
comes in contact with the limestone on the north, the latter for a 
few feet appears confused, is not stratified, contains many large 
cavities coated with bitter spar, and is intimately united with 
numerous small angular fragments of quartz, which are often 
exposed on the surface from the weathering of the rock.* 
* I am aware that Dr. Mac Culloch has given a different view of this tract, since this 
paper was presented to the Society, but having again visited the spot I am inclined to 
think that he has taken his ideas from supposing the portion of limestone marked ( b ) to 
extend further than it really does. Perhaps the impossibility of visiting this rock, which 
can only be approached at very low tide, may have led to the error. 
