12 PHILOSOPHICAL TRANSACTIONS. [^ANNO 1791- 



granites, with large grains, and composed of nearly equal quantities of white 

 feidspath and black shoerl. The transition depends altogether on an increased 

 proportion of feidspath and on the enlargement of its grains ; a phenomenon 

 which leaves no room to doubt, that all these stones belong to the same system 

 of mountains." 



By observations like these, which the specimens Dr. B. either possesses, or has 

 examined, corroborate and complete, he is persuaded, that when once it becomes 

 an object of attention, persons who have an opportunity of exploring countries 

 where basaltes and granite abound, will easily find a succession of specimens 

 beginning at the former and terminating at the latter. Nor is it perhaps difficult 

 to assign highly probable reasons, why a mixture of different earths with more or 

 less of metallic matter, in returning from a state of fusion to a solid consistence, 

 may assume sometimes the homogeneous basaltic, and sometimes the heteroge- 

 neous granitic internal structure. No fact is more flimiliar than that it depends 

 altogether on the management of the fire, and the time of cooling, whether a 

 tnass shall have the uniform vitreous fracture, or an earthy broken grain, arising 

 from a confused crystallization. The art of making Reaumur's porcelain consists 

 entirely in allowing the black glass time to crystallize by a slow refrigeration ; and 

 the very same mass, according as the heat is conducted, may, without any altera- 

 tion of its chemical constitution, be successively exhibited any number of times as 

 glass, or as a stony matter with a broken grain. In the slag of the iron furnaces, 

 the same piece generally exhibits both these appearances ; the upper surface cools 

 fast, and is glass ; what lies deeper, loses its heat more gradually, and is allowed 

 time to take on the crystalline arrangement peculiar to its nature, in as far as a 

 number of crystals, starting from various points at once, and crowding each 

 other, will admit of it. Here indeed the crystals are uniform, and not of a differ- 

 ent form and composition, as in granite; so that this analogy applies closely only 

 to basaltes; and it perfectly explains why this body in congealing has assumed an 

 earthy and not a vitreous grain. But it is easy to conceive how, under certain 

 variations of heat and mixture, a melted mass may coagulate into quartz, feidspath 

 and shoerl, or mica. The most permanent difference between basaltes and 

 granite, as to mixture, consists in the quantity of iron ; for the earths in the in- 

 numerable varieties of each vary indefinitely in their proportions ; and as to heat, 

 that the latter having been perhaps in general raised from a greater depth, and 

 consisting of more' huge masses, must have cooled more slowly, and perhaps they 

 have undergone different degrees of fusion. Besides toadstone, basaltes inclosing 

 feidspath, zeolite, See, various lavas clearly demonstrate that heterogeneous 

 earthy crystals do separate from a fused paste, once undoubtedly as uniform as a 

 metallic calx, and its reducing flux before the subsidence of the metallic particles. 

 We shall probably be much deceived by a narrow analogy if, because in our pro- 



