w> Mr. Mills's Obfervations on 
derived) Handing vertically, and appearing to fill up the chafms 
formed at fome remote period in the ftrata. 
In giving a more minute account of the Whyn Dykes which 
have fallen under my obfervation, it becomes neceffary to men- 
tion, that when I arrived at Loch Loudain, I there hired a 
row boat, in which I proceeded to Freeport Bay* At the place 
where I re-imbarked, the fhoreis low, and compofed of horn- 
ftone. After paffing the ruins of an ancient caftle, in about 
an hour and an half I landed on a rocky point, where I difco- 
vered a large Whyn Dyke, between three and four yards wide, 
Handing vertically, and ranging S. W. into the land* It is of 
a dark-brown colour, externally approaching to black, and 
contains bladder-holes, fome of which are filled with quartz, 
and others with reddifh lamellar fpar, which effervefces ftrongly 
with nitrous acid, as do many parts of the bafis of the fub- 
itance, which is too foft to flrike fire with Heel, but is confide- 
rably magnetic, and of the fpecific gravity of 2,863. Embarking 
from hence, and eroding a fmall fandy bay (named Thu rot’s 
Bay, from that gallant Frenchman having landed there), we 
reached the high mountainous point called McArthur’s 
Head, which forms the S.E. point of the found of Ilay. The 
ftratum here is white chert, with many large patches and 
{tripes of red, which indicate that it contains iron ; but the 
great height, and almoft perpendicularity, of the cliffs, pre- 
vented my minutely examining them. From thence to Port- 
aikeg and Freeport Bay, the rocky fhore, which is compofed 
of chert, is lefs elevated, fhews many indications of iron, and 
has feveral fiffures and caverns ; and in fome places has Whyn 
Dykes burfting through it, which appear tore-enter on the 
oppofite cherty fhore of Jura. 
About 
