10 PHILOSOPHICAL TRANSACTIONS. [aNNO 1791. 



path and grains of transparent quartz are difFused through a paste of the same 

 brownish red colour and texture as the basaltic columns at Dunbar in Scotland. 

 Nothing is indeed more common, or more variously modified, than fossils of 

 this intermediate character. We frequently find a ground of jasper, and no 

 doubt also of different varieties of whinstone, as will hereafter appear, with 

 feldspath and shoerl at the same time imbedded in them ; and again with grains 

 of feldspath and quartz in such a manner as to leave it extremely doubtful, 

 whether the rock ought to be named granite or porphyry. The varieties of 

 such rocks will conduct us, by easy steps, from uniform basaltes through the 

 porphyries to granite. A chemical examination of the basis of a number of 

 these porphyries would be very interesting; yet he would not rest the theory of 

 their formation altogether on the result of analysis. The same stratum is per- 

 petually varying in its mixture; and we should not too rigorously adhere to the 

 proportion of ingredients discovered by the chemist in the hundred grains on 

 which his experiments may chance to be made. The sensible qualities, the stile 

 of fissure, the accompanying fossils, and the form of whole rocks, when sur- 

 veyed by an experienced eye, are as good criterions of basaltes as a certain pro- 

 portion of iron, and the black glass which it yields on fusion. Should the matter 

 of any given rock contain too little iron to be fusible by the blow-pipe, and yet 

 have other striking features of whinstone, would this be a sufficient reason to 

 conclude that its formation has been different? Chemistry, if thus strictly fol- 

 lowed, would perplex mineralogy, instead of reducing it to order. Cliaracters 

 of minerals, purely chemical, would separate those whose natural history is alike, 

 and bring together such as differ widely in their formation. 



The late Mr. Ferber's letters from Italy furnish so many facts, conspiring in 

 one way or another to show the affinity between basaltes, as well as other pro- 

 ducts of subterraneous fire and granite, that whoever reads them with this view, 

 will doubtless find himself more interested and instructed. The following are 

 among the most striking of these facts. 



" 4th species of basaltes. Oriental basaltes through which the constituent 

 parts of granite are equally diffused. Separate particles of red feldspath, quartz, 

 and mica, are dispersed through the substance of this species: they seem to have 

 been distributed through an aqueous solution, and to prove, that this species had 

 rather an aqueous than a fiery origin." I see neither proof nor presumption in 

 favour of this supposition; but in a series of specimens, collected with a view 

 to show the transition from black basaltes to granite, this species and the granite 

 from Esterelles would form two contiguous links. " 5th Oriental basaltes, with 

 stripes of granite. The common black basaltes, fasciated with large stripes of 

 red granite, blended and joined to the basaltes without any visible separation; not 

 as the pebbles in a breccia, or as fissures healed up and filled with granite, but as 



