on Basalt , &c. 
2S3 
conchoidal fractures, which display the form of each globule. 
But, if the arrangement has extended a little farther, all these 
subdivisions are entirely lost ; the mass becomes perfectly com- 
pact, has an even or a flat conchoidal fracture, is nearly of the 
same hardness as the glass, is commonly of a chocolate colour, 
graduating into a brownish-black, and the intensity of the 
colour increases in proportion to the degree to which the ar- 
rangement has extended. Its aspect is rather greasy ; and 
it much resembles some varieties of jasper, in the compactness 
of its texture, and in its opacity. Its magnetic action is ex- 
tremely feeble. Its specific gravity appears to be 2.938. 
3d. If the mass were now rapidly cooled, it is obvious that the 
result would be the substance I have ju§t described ; but, if the 
temperature adapted to the farther arrangement of its particles 
be continued, another change is immediately commenced, by 
the progress of which it acquires a more stony texture, much 
greater tenacity, and its colour deepens as these changes advance, 
till it becomes absolutely black. Sometimes this alteration is 
effected by a gradual transition, the limits of which cannot be 
assigned, but more generally by the formation of secondary 
spheroids, in the heart of the compact jaspideous substance. 
These spheroids differ essentially from those first described ; the 
centres of their formation are more remote from each other, and 
their magnitude is proportionably greater, sometimes extend- 
ing to a diameter of two inches, and seeming only to be limited 
by contact with the peripheries of other spheroids. They are 
radiated, with distinct fibres ; . sometimes the fibres resemble 
those of brawn haematites, and sometimes they are fasciculated 
irregularly, so as to be very similar in appearance to the argil- 
laceous iron ores rendered prismatic by torrefaction. They are 
