4^9 



miK, or the parallel ftrata in the leaft to interfere with each other, 

 a nicety of operation utterly unattainable in the laboratories of our upper 

 regions. ,. 



I once thought, that when I found, at Portrup and its neighbour- 

 ing iflands, bafalt ftrata abounding with marine {hells, that I had got 

 convincing proof bafalt was not of igneous origin ; T was not then 

 acquained with the powers of Dr. Button's fubmarine laboratory, nor 

 did I know that he could there fufe fubftances, which in our fires 

 are calcinable or combuflible ; but he exprefsly tells us, (page 282), 

 " that if the theory now given be jull, a rock of marble is no lefs 

 " a mark of fubterranean fire and fufion, than that of the bafaltes."* 



Mr. Playfair is more particular, and, as the reader probably never 

 faw limeftone expofed to violent heat without calcining, nor coal with- 

 out burning, he may be glad to know how thefe refraftory fubftances 

 may be fufed like metals, and melted like wax. 



The great agent employed, for this purpofe is prejfure, whofe powers, 

 by Mr, Plavfair'% account are fo very extraordinary, that for 

 fear of mifreprefentation I fliall carefully ufe his own words : he fays, 

 (feftion 132,) " Ihe circumftance which gives Dr. Huttonh theory its 

 " peculiar charafter, and exalts it infinitely above all others, is the 

 " introduftion of the principle of preflure,, to modify the efFefts of heat 

 " when applied at the bottom of the fea, (fee 15) this important 

 " remark was firft made by Dr. Hutton, and applied with wonderful 



fuccefs, 



* I wifli Dr. Hutton had been fo good as to tell us what marks o£ _fire and fu- 

 fion a rock of marble exhibits ; the moft ftriking circumftance that occurs on the in- 

 Ipedlion of a piece of marble is, that it generally abounds with marine (hellsi with 

 their diftinft forms accurately preferyed ; with us, in our limited, fuperficial ex- 

 perience, the invariable effeft of fufion is the obliteration of all forms, interior or ex- 

 terior, pofleffed by the mafs before it was expofed to the fire : I prefume the power of 

 fufing without effacing forms, is one more of the many advantages which Dr. Hutton s 

 fubaqueous laboratory poiTeires over ours ; ftill he leaves us in the dark as to the 

 agent, whether it was his own prefTure, or the marine acid, employed by M. St. Fond, 

 on fimilar occafions ; pofTibly, had he not been in a hurry, he would have introduced 

 lis to fome new agent equally powerful. 



