OBSERVATIONS ON BASALT, &C. 117 



two of (he fpheroids come into contacl by mutual enlargement, 



no intermixture of their fibres feem to take place; they ap- Contact with 



pear equally impenetrable, and, as neither can P enetrate >d^°'^P h r °* 



both are comprefTed, and their limits are defined by a plane, drons. 



at which a feparation readily takes place, and each of the 



fides is inverted with a rufty colour. When feveral fpheroids 



come in contact on the fame level, they are formed by mutual 



prelTure into pretty regular prifms, whofe divifion is perfectly 



defined; and when, a fpheroid is furrounded on all fides by 



others, it is comprefTed into an irregular polyhedron. 



4th. The tranfition from this fibrous ftate to a different ar- Third arrange- 

 rangement, feems to be very rapid ; for the centre of mod offtony" texture? 

 the fperoids becomes compacl, before they attain the diameter great tenacity, 

 of half an inch. As the fibrous figure propagates itfelf by X ke ^™^"" 

 radiating into the unarranged mafs, the compacl nucleus which aftion. 

 fupplies its place gradually extends, till it finally attains the 

 limits of the fpheroids ; and the fame arrangement pervades 

 the matter comprehended between them. The mafs has now 

 afllimed a compacl ftony texture, and poffeffes great tenacity. 

 Its hardnefs is fomewhat inferior to that of the glafs from which 

 it was formed. Its aclion on the magnetic needle is very con- 

 siderable. Its fpecific gravity is 2.938. Its colour is black, 

 inclining to fleel gray: it is abfolulely opaque, and only re- 

 flecls light from a few minute points. Though the divifions 

 between the fpheroids are rendered imperceptible to the eye, 

 they are not obliterated, and their rufty furfaces are often dif- 

 clofed by an attempt to fracture the mafs. 



jth. A continuation of the temperature favourable to ar- Granular tex- 

 rangement, fpeedily induces another change. The texture ture ' 

 of the mafs becomes more granular, its colour rather more 

 gray, and the brilliant points larger and more numerous: nor 

 is it long before thefe brilliant molecules arrange themfelves 

 into regular forms; and, finally, the whole mafs becomes per- 

 vaded by thin cryftalline laminae, which interfecl it in every Cryftalline lami- 

 diredion, and form projecting cryftals in the cavities. The n «« 

 hardnefs of the bafis feems to continue nearly the fame; but 

 the aggregate aclion of the bafis, and of the imbedded cryf- 

 tals, on the magnetic needle, is prodigioufly increafed. It Polarity, 

 appears to polfefs fome polarity; and minute fragments are 

 fufpended by a magnet. Its fpecific gravity is fomewhat in- Increafed den- 

 crealed, as it is now 2.949, The cryftals contained in it, flt y* 



when 



