Mr. Mills’s Obfervation-s oh 
88 
LETTER IT. 
BEAR SIR, Fence Houfe, Feb. I, 1789. 
HAVING in my former letter (hewn, that Whyn Dykes, 
or, in other words, veins of lava, are found in the vicinity of 
columnar bafaites, which latter are now, by almoft univerfal 
confent, acknowledged to be of volcanic origin ; I (hall pro- 
ceed to defcribe the Whyn Dykes of Hay, and (hall commence 
with giving you a general idea of the extent and various ftrata 
of that ifland. 
Ilay, from the northern to the fouthern extremes, is about 
thirty miles in length, and in one part extends nearly as much 
in breadth from the eaftern to the weftern fhores. The S.W. 
part of the ifland is low ; but the land rifes to the N.E. and 
the mountains and cliffs which form the north-eaft coaft are 
fteep and lofty. At the head of Loch-in-Daal, and near Bo- 
more, the rock which appears at day is of curled hornffone, 
and does not feem to have any particular dire&ion. Purfuing 
the road (N.E.) to Port Afkeg, the hornffone becomes more 
regularly laminated, and ranges N.W. and S.E. ; and fome of 
the hollows between the little riling hills are found to contain 
gravel. At four miles from Bomore commences the limeftone 
ftratum, the joints in which are nearly vertical, and range in 
general N.E. and S.W. It is about four miles in breadth 
from S.E. to N.W. ; but I have not been able to difcover how 
far it extends in the courfe of its range, never having traced it 
more than three miles in that direction ; but it is nearly certain, 
that it does not reach either the eaftern or the weftern fhore, 
from 
