116 OBSERVATIONS ON BASALT, &C. 



homogeneous body, wholly unlike glafs, and equally unlike 

 the parent bafalt. When the union of the little globules has 

 been imperfectly effected, the fracture of the mafs indicates 

 its fiructnre, by numerous minute conchoidal fractures, which 

 difplay the form of each globule. But, if the arrangement has 

 extended a little farther, ail thefe fubdivifions are entirely loft j 

 the mats becomes perfectly compact, has an even or a flat con- 

 choidal fracture, is nearly of the fame hardnefs as the glafs, 

 is commonly of a chocolate colour, graduating into a brownifh- 

 black, and the intenfity of the colour increafes in proportion to 



It rcfembles jaf- the degree to which (he arrangement has extended. Its afpecl; 



rttl' magnetic '* rather greafy ; and it much refembles fome varieties of jafper 



aclion. in thecompactnefs of its texture, and in its opacity. Its mag- 



netic action is extremely feeble. Its fpecific gravity appears 

 to be 2.938. 



More advanced 3d. If* the mafs were now rapidly cooled, it is obvious that 



theTon C tc^ lhe lefult WOll!d be the rilbftance * have J llft defcribed; but, 

 ture. ' if the temperature adapted to the farther arrangement of its 



particles be continued, another change is immediately com- 

 menced, by the progrefs of which it acquires a more ftony 

 texture, much greater tenacity, and its colour deepens as 

 Darker colour, thefe changes advance, till it becomes abfolutely black. 

 Sometimes this alteration is effected by a gradual tranfition, 

 the limits of which cannot be affigned, but more generally 

 by the formation of fecondary fpheroids, in the heart of the 

 compact jafpedeous fubftance. Thefe fperoids differ effen- 

 liaily from thofe firft defcribed ; the centres of their formation 

 are more remote from each other, and their magnitude is pro- 

 portionably greater, fometimes extending to a diameter of 

 two inches, and feeming only to be limited by contact with 

 Large fpheroids the peripheries of other fperoids. They are radiated, with 

 tL" d ' ated CeX " diftin & fibres * fometimes the fibres refemble thofe of 

 brown haematites, and fometimes they are fafciculated irregu- 

 larly, fo as to be very fimilar in appearance to the argilla- 

 ceous iron ores rendered prifmatic by torrefaclion. They are 

 generally well defined, and eafily feparable from the mafs 

 they are engaged in ; and often the fibres divide at equal 

 diftances from the centre, fo as to. detach portions of the 

 fperoid in concentric coats. The tranfverfe fracture of the 

 fibres is compact and fine grained ; the colour black ; and the 

 hardnefs fomewhat inferior to that of the bafaltic glafs. When 



two 



