262 Proceedings of the Royal Physical Society. 



anorles to the coolinsj surface. Of course, the walls of vitrified 

 forts would first be built of rubble without mortar, in the 

 same manner as dry stone dykes. They would in this way 

 contain examples of most, if not of all, the stones in the district 

 where they were erected. On the application of heat all the 

 stones which contain soda, potash, lime, baryta, etc., would 

 slag or vitrify. I think it can be made good that in the case 

 of Knockfarrel, the vitrifying process had been applied to the 

 building at different stages in the rearing of the wall. What, 

 however, seems to me of most value here is, that these forts 

 present appearances which shed a good deal of light on 

 phenomena which every field geologist has to deal with. Thus, 

 the influence of heat on some sandstones met with in the 

 walls of these forts is precisely similar to what meets us in 

 nature. Here is a bit of sandstone from Hailes Quarry in the 

 neighbourhood, which exhibits, in a most marked way, the 

 thin layers of which it is composed. Now, place this along- 

 side of the gneiss from Stonehaven, the specimen from 

 Muchalls, Kincardine, marked by Fleming "gneiss passing 

 into mica schist," and the two or three specimens on the 

 table from the Finhaven vitrified fort, and it will be almost 

 impossible to miss the meaning of these in connection with 

 the influence of heat on the altered specimens, or to mistake 

 the steps of their geognostic history. Again, a close examina- 

 tion of the examples from the several forts shows how deep 

 the prism form penetrates. In one part of a Finhaven speci- 

 men minute prisms lie at right angles to the top of the stone, 

 in a way to suggest the impress which a ribbed hemlock 

 stalk, or the parallel venation of the leaf of some low plant, 

 would leave on soft clay. But in another part of the same 

 specimen the prisms present a resemblance in miniature to 

 the deep foliated structure of Psilomelane, one of the ores of 

 manganese, now on the table. A specimen from Knockfarrel 

 presents several tiny tiers of prisms, arranged after the manner 

 of the gigantic columns of Staffa and the Giant's Causeway. 

 I was much interested at Knockfarrel in the occurrence, here 

 and there, on the stones of small semi-opalesque blisters. It 

 was difficult to detach a good specimen, but after many careful 

 attempts the specimen now shown was obtained. Are we to 



