ON PORPHYKT. 



i-y and red sandstone in the island of Arran, aM 

 frequently in the form of veins traversing the 

 sandstone. 



I have given an account of the occurrence of 

 porphyry in floetz country, in my Description of 

 the Island of Arran, published in the year 1798, 

 and in my Mineralogical Travels published in the 

 year 1 800. In my System of Geognosy, publish- 

 ed in the year 1 808, I do not introduce the Floetz 

 Porphyries into the Wernerian arrangement of 

 rocks there explained, because I was not then con- 

 vinced of the accuracy of my early observations. 

 Having again visited Arran, and re-examined the 

 appearances described in my early writings, I find 

 that they are correct ; and, therefore, do not now 

 hesitate to consider Porphyry as a member of the 

 Flostz class. 



