Prismatic Structure in Stones of Scottish Vitrified Forts. 261 



the mode of vitrifaction. These may be held to exhaust the 

 literature having direct bearing on the vitrified forts of Scot- 

 land. There is not the slightest reference in any of these 

 papers to the prismatic structure in the artificially-calcined 

 stones. About fourteen years ago I was much struck by it 

 when on a visit to the vitrified fort at Finhaven, Forfarshire, 

 in company with the late Mr Crichton, writer, Forfar, on 

 the farm of whose father the fort stood ; tlie Piev. Eichard 

 Waterstone, Free Church minister, Forfar, now of Glasgow ; 

 and Mr James K. Martin of Bridgehouse, now of Blackburn 

 Mill, Linlithgowshire. At that time the facts of prismatic 

 form were so manifest, that I concluded they must be well 

 known to observers. It was only when a small fragment was 

 shown to the Society, on the occasion referred to above, that 

 it was made clear it was very little known. I then showed 

 some of my Finhaven specimens. So recently, however, as 

 about ten days ago, I found that the late Professor Fleming, 

 my predecessor in the New College Natural Science Chair, 

 had observed the prisms in these stones. When in search of 

 some minerals which had been misplaced, I found the speci- 

 mens now shown which had been marked by Fleming. 



The occurrence of prismatic structure in rock masses is 

 well known. For example, in some of the low trap hills near 

 Torphichen, Linlithgowshire, it can be seen even on the sur- 

 face of the rock, suggesting a natural kind of mosaic. Almost 

 every fragment broken off by the hammer, and even minute 

 pieces dislodged by weathering, present unsymmetrical penta- 

 gons. It is not uncommon in other volcanic rocks, and I have 

 seen it even in coal, at points where a trap dyke penetrated the 

 seam. The correspondence of natural volcanic with recent 

 lavas might be very fully illustrated. I now show to the 

 Society a mass of lava from Etna, showing a very distinct 

 tendency to prismatic structure. If the grain of this, and of 

 another specimen from Vesuvius, be compared with two 

 examples of porphyrite from Dunack, near Oban, the close 

 resemblance will at once appear. And, looking at basalt, it 

 should be remembered that the prismatic structure is not to 

 be traced to crystallisation, but to the fact that compound 

 melted bodies, when cooling, form prisms in a line at right 



