39 



fufion, (page 253). Dr. Hutton however confefles it is not eafy to com- 

 prehend tliis : " and to be convinced that this calcareous ftone, which 

 " calcines fo eafily in our fires, fliould have been brought into fufion by 

 " fubterraneous heat without fuffering calcination, mud require a chain 

 " of reafoning ivhich everyone is notable to comprehend." (Page 271.) 



But it is not neceffary on this occafion to enter into the myfteries of a 

 laboratory, to which we have not accefs, nor to calculate the force of 

 Dr. Hutton's great agent comprejjlon ; for our obfervations on the con- 

 tafts of the matter of our Whynn Dykes with the fubflances they en- 

 counter, being made on the furface of the earth, in the open air, even 

 admitting thofe Dykes to be formed as Dr. Hutton fuppofes, his un- 

 erupted lava is now become erupted, and of courfe, to ufe his own words, 

 ■** thofe fubflances which calcine and vitrify in our fires, fliould fuiFer 

 " fimilar changes when delivered from a comprefllon which renders thein 

 " fixed." (Edinburgh Tranf. page 280.) 



I am aware I mufl: fatigue your Lordfliip by dwelling fo long upon 

 the queflion of the igneous origin of our Whynn Dykes ; but as moil 

 modern writers and travellers call them lava •veins, and the faBs I have 

 obferved with much attention, induce me to combat fo general and fo 

 popular an opinion, I hope you will excufe me for adding a fourth 

 argument, which I conceive to be conclufive. 



All fubflances, when ignited, are in an high flate of dilatation ; this is 

 followed, when they cool, by a contraflion, line rctraite, by which they 

 occupy lefs fpace than they did when heated ; of courfe, had our Dykes 

 been chafms filled up with glowing lava, when this material cooled and 

 contrafted, it could no longer fill up thefe chafms as before, but mufl 

 crack and fepanitc from their fides, leaving intervals and difruptions ; but 

 nothing like this is obferved, the Dyke and contiguous matter, whatever it 

 be, are folidly united together, forming but one mafs. 



Thefe Whynn Dykes fuggefl: other curious queflions: Were they form- 

 ed at the fame time with the contiguous materials? 



Were they pofterior to them as Dr. Hamilton and Dr. Hutton fuppofe? 



Or 



