on Basalt, See . 
281 
arrangement of bodies passing from a vitreous to a stony state, 
which might have remained unobserved, if the desired homo- 
geneity of the result had been obtained. I shall now endeavour 
to describe the various products of this operation ; and I shall 
also submit to your consideration, some remarks which appear 
to me to arise naturally from the phenomena I have observed ; 
premising that, except where my opinions are supported by the 
unequivocal demonstration of facts, I offer them with the utmost 
deference to the decision of more experienced and judicious 
mineralogists and geologists. 
It may be proper to give a concise description of rowley rag 
itself, before I consider the products which it yields by igneous 
fusion. This species of basalt is fine-grained, of a confused 
crystallized texture; its fracture uneven in small pieces, con- 
choidal in large pieces. Its hardness superior to common glass, 
but inferior to feldspar. Its tenacity considerable. Its action on 
the magnetic needle strong, but without signs of polarity. Its 
specific gravity, according to my trials, 2.868. Its general 
colour iron gray, approaching to black. It is opaque ; and it 
reflects light from a number of brilliant points, some of which 
seem to be feldspar, and others hornblende.* 
* “ The ragstone has been -accurately analysed by Dr. Withering, who found 
<c that 1000 parts of it contained 475 parts of siliceous earth, 325 argillaceous earth, 
“ and 200 calx of iron; but this iron seems to me to be in a very small degree of 
“ calcination, from the dark blue colour of the stone, from the rusty colour it assumes 
“ on being exposed to a farther state of calcination by air and water; and from the 
“ magnetic property of the mountains, which, as Dr. Plot observed, turned the needle 
“ 6° from its proper direction. This magnetic property has since been observed in 
“ several basaltic mountains, particularly in the Giant’s Causeway in Ireland, and 
« very remarkably in a basaltic columnar mountain called Compass Hill, in the island 
O O 2 
