OBSERVATIONS ON BASALT, &C. H5 



reflects light from a number of brilliant points, fome of which 

 feems to be feldfpar, and the others hornblende. * 



lft. This fubftance is eafily fufed into glafs, whofe texture is Its glafs. 

 completely vitreous, with few air-bubbles. Its fracture undu- 

 lated conchoidal. Its hardnefs fuperior to feldfpar, but inferior 

 to quartz. It poftefles fcarcely any aclion on the magnetic 

 needle. Its colour is black : it is nearly opaque, being tranf- 

 lucent only in very thin fragments. Its fpecific gravity ap- 

 pears to be 2.749. 



2d. The tendency towards arrangement in the particles of Firft appearance 

 the fluid glafs, is firft developed by the formation of minute s ma ii globules * 

 globules, which are generally nearly fpherical, but fometimes appear in the 

 elongated, and which are thickly difleminated through the ma 8 * 

 mafs. The colour of thefe globules is confiderably lighter than 

 that of the glafs; they are commonly grayiih-brown, fome- 

 times inclining to chocolate brown, and, when they have been 

 formed near the interior furface of the cavities in the glafs, 

 they project, and refemble a clufter of fmall feeds. Their 

 diameter rarely exceeds a line, and feldom attains that fize, 

 as, in general, they are fo near to one another, that their 

 furfaces touch before they can acquire confiderable magnitude. 

 In the procefs of cooling, they adapt their form to their con- 

 fined fituation, fill up every interftice, and finally prefent a 



* " The ragftone has been accurately analyfed by Dr. Withering, Analyfis of row- 

 who found that 1000 parts of it contained 475 parts of fdiceous lev . ra ? b y Dr « 

 earth, 325 argillaceous earth, and 200 calx of iron j but this iron Wltherm S« 

 feems to me to be in a very fmall degree of calcination, from the 

 dark blue colour of the ftone, from the rufty colour it aflumes on 

 being expofed to a farther ftate of calcination by air and water, and 

 from the magnetic property of the mountains, which, as Dr. Plot 

 obferved, turned the needle 6° from its proper direction. This 

 magnetic property has fince been obferved in feveral bafaltic moun- 

 tains, particularly in the Giant's Caufeway in Ireland, and very 

 remarkably in a bafaltic columnar mountain called Compafs Hill, 

 in the ifland Cannay, one of the Hebrides, defcribed by George 

 Dempfter, Efq. in the Tranfaclions of the Society of Antiquaries 

 in Scotland, Vol. I." See Mineralogy of the South-weft part of 

 Staffordmire, by James Keir, Efq. F. R. S. published in Shaw's 

 Hiftory of Staffordmire, Vol. I. 



Mr. Kirwan Mates the fpecific gravity of rowley rag, which he 

 calls ferrilile, at 2.748 5 and affigns its melting point at 98* of 

 Wedgwood's pyrometer, 



I 2 homogeneous 



