468 Geological Society. 



question of accident or design might be illustrated, by exa- 

 mining in the laboratory the degree ot heat required to pro- 

 duce the appearances in the stones which actually existed in 

 these structures. 



The fort of Dim MacSniochain stands on a long narrov^ 

 hill, which is nearly precipitous along three parts of its cir- 

 cumference, and at the other end it rises from the plain 

 with a very accessible acclivity. The walls, which are all 

 at present buried under the soil, are at)out eight or ten fectf 

 in thickness. They bear marks of vitrification through 

 their whole extent; but in no case does it appear to have 

 exl ended more than a foot or two upwards, and the most 

 J erfect slags are found at the bottom of the foundation. In 

 the higher parts of ihesc are stones roasted by the action of 

 the heat, but unvitritlcd, and at length the marks of fire 

 almost entirely disappear. The hill consists of alternate 

 heds of schistus and limestone, bul the latter is the predo- 

 minant rock. 



It is perfectly insulated in a great alluvial plain. The 

 moumtains of Bcnediraioch, which bound the plain to the 

 west, consist of granite gneiss, mica-slate, quartz and por- 

 phyry. On the edge of these rocks are found large de- 

 tached masses of imddintrstone, consisting of rounded 

 pebbles of greenstone of different varieties, of amygdaloid 

 and quartz cemented by a paste which appears to consist 

 chiefls' of trap sand, united by the hard variety of calca- 

 reous spar. The paste contains also in small quantity zeo- 

 lite, prchnite, garnet, and diallage. This puddmgstone 

 where nearest the fort is at least half a mile distant from 

 it. The walls of the fort consist principally of granite 

 cneiss, mica-slate, clay-slate quartr, puddingstone and py« 

 rilical slate entangled together with a verv sntall j)roporlioa 

 of the particular rock on which the fort itself is founded: 

 puddingstone forming the greater part of them. 'J'his pud- 

 dingstone Dr. M. shows to be the only viirifiable ingre- 

 dient of the walls ; and from the tli^iance from which it 

 must iuivc been brought, and the great quantity of it em- 

 ployed in the work, lie considers it probable that the build- 

 ers ot the fort inu^l have been acquainted with its vitrifiable 

 nature, and that it >.vas on account of this quality that they 

 had employed so groat labour in transporting it. For if 

 iheir object h ui not been to produce vitnilcation, but mere- 

 ly to erect a dry wall of stone, the limestone of the hill 

 would liave answcicd their intentions, or perhaps the loose 

 s»c)nos of the adjoining plain. 



That they did not obtain the puddingstone from the 



latter 



