io6 Pumice-fane and Bafi&es. [Book VI. 



crater, and broader and fhallower as they advance, 

 unlefs fame valley intervenes. Pumice-ftones lie at 

 a ftill greater diftance ; and from thefe obfervations, 

 fays Mr. Kirwan, extingiiifhed volcanoes may be 

 traced. The quantities of matter thrown out of vol- 

 canoes at one eruption, are often fo great as to cover a 

 ipace of country of many miles, and to be many years 

 in cooling. 



II. PUMICE-STONE is a volcanic ejection, but is 

 frequently found at a diftance from its origin. Its 

 colours are grey, white, and reddim brown. It is 

 hard, rough, porous, confifts of (lender fibres parallel 

 to each other, is very light, and with difficulty gives 

 fire with ftecl. It feems to have been originally an 

 aibeflos decompofed by the action of fire. One hun- 

 dred parts contain from fix to fifteen of magnefia, with 

 a fmall portion of calcareous earth -, the remainder is 

 chiefly filex. Pumice- flone fwims on water. It is 

 ufed to fmooth rough furfaces, and, in a flate of pow- 

 der, in various branches of manufacture, chiefly for 

 polifhing. 



III. BASALTES is a flone of a dark grey colour, co- 

 vered with a ferrugineous cruft, and generally cryf- 

 tallized in opake triangular or polyangular columns. 

 When it is amorphous *, and breaks into large, thick, 

 fquare pieces, it is called trapp. When heated red 

 hot, and quenched in water, it becomes by degrees of 

 a reddifh brown. It melts without mixture into a 

 perfect flag. One hundred parts contain fifty-two fiii- 

 ceous, fifteen argillaceous, three calcareous, two of 

 magncfian earth, and twenty-five of iron. Bafaltes 



* Not of a regular form, 



fometimes 



