4S5 



In like manner Mr. Playfair fuppofes his lava was injefted be- 

 tween the ftrata of this Portrufh ftone with fhells ; and at once 

 confidering the fadl as eftablifhed, he proceeds to avail himfelf of 

 it, and to account for fome fimilar appearance in the illand of 

 Ceriga, which he fays, " The Italian Naturalift {Spalanzani) fuppofes 

 *' to be of volcanic origin." 



Thefe he admits would have been embarraffing, " without the 

 " commentary afforded by the Portrulh fpecimens." From thefe 

 he concludes^ " that in both cafes the fhells are involved in parts of 

 " the rock, which have been in fome degree aflimilated to the 

 " bafalts, by the heat they have endured." — " Spalanzani would pro. 

 " bably have ufed exaftly the fame terms which he employs in 

 " fpeaking of Cerigo, if he had been required to defcribe the petri. 

 " fied fliells at Portrujh." (Page ftSg.) 



Whether Spalanzani would have been equally ready to have ac- 

 commodated the Ptrtrujh fads to his theory (a very different one 

 from Dr. Hution's) I will not take upon me to determine ; but 

 unfortunately they are mofl perverfely contrary to what Mr. Playfair 

 afferts (pages 287, 288) : " Upon the whole it is evident, that the 

 " rock containing the fhells has acquired a high degree of indu- 

 ** ration by the vicinity of the great ignited mafs of whyn flone." 



No doubt fo it ought, according to Mr. Playfair's Theory ; and 

 he niufl have thought it no great flretch to afTert, that things 

 adually were, as he thought they ought to be. 



Unhappily the very contrary is the fad ; for, inftead of this flra- 

 tified flone being affefted " by the vicinity of the great ignited 

 " mafs of whyn ftone," the alteration at the contact is made upon 

 the whyn flone alone, which grows fomewhat finer as it approaches 

 the line of demarcation ; and what makes this more provoking is, 

 that it is the fingle inltance in which I have found any flratum af- 

 fefted by the contiguity of another. 



The 



