1817.} On the Geology of South Wales. 115 
stone containing 38 per cent. of carbomate of magnesia. One of 
the places to which I allude is the high ridge of ground which runs 
east and west to the south of Caerphylly, about four miles north of 
Cardiff. I walked for a considerable distance along this ridge, and 
still found no appearance of the true mountain lime-stone ; and as 
the range appeared to run for many miles east and west in a con- 
tinuous line, it is probable that the magnesian lime-stone will be 
found to occupy a considerable part of its course. The next place 
to which I allude is the hill which overhangs Risca in coming from 
Machen, the greater part of which is composed of the same variety 
of magnesian lime-stone, and is there most distinctly immediately 
subjacent to the coal measures which make their appearance, I 
believe, even in the village of Risca. This variety of magnesian 
lime-stone is of a grey colour, hardly different from that of the 
usual varieties of mountain lime-stone ; but, independent of chemical 
criteria, it is readily distinguished from that rock by its greater 
specific gravity, by its greater hardness, and by its possessing a 
highly glimmering lustre : whereas the mountain lime-stone, when 
free from scales of calc spar, is generally quite dull. ‘The observa- 
tions of Mr. Tennant had established the opinion that magnesian 
lime is injurious to land, when used in considerable quantity; but 
we learn from Dr. Thomson, that this kind of lime is used as a 
manure in the North of England, with very little caution, and that 
it is productive of no bad consequences. ‘The same is the fact 
in the districts to which I have alluded. I found the hill at Risca 
and the ridge south of Caerphylly crowned with quarries and lime- 
kilns, and was informed that the lime is very extensively employed 
for agricultural purposes, without any idea of its having properties 
different from those of common lime. A quarryman however was 
aware that this stone took a longer time to burn than common 
lime-stone, as for instance the lyas. 
It is probable, when this species of magnesian lime-stone shall 
have become more generally known, that it will be found frequently 
associated with the mountain lime-stone ; but in colour it so much 
resembles that rock, that few would at first sight be led to suspect 
any thing remarkable in its chemical composition, 1 discovered 
some time ayo precisely the same variety of magnesian lime-stone 
in the neighbourhood of Bristol interstratified with the mountain 
lime-stone, and have lately transmitted an account of it, together 
with its analysis, to the Geological Society. I have, besides, met 
with a more crystalline species lying upon the mountain lime-stone 
near Ross; and Mr. Wm. Clayfield showed me a specimen of the 
mountain magnesian lime-stone collected by Mr. Tennant, I believe 
from the Mendip Hills. 
2. In former communications I had hazarded the conjecture that 
the old red sand-stone will generally be found under the first flogtz 
or mountain lime-stone, and wherever 1 met with the latter rock 
in South Wales this was always the case. At Risca, and in the 
range below Caerphylly, the magnesian lime-stone, which is the 
H 2 
