SI& SCIENTIFIC NEWS* 



quartz and porphyry. On the edge of these rocks are found! 

 large detached masses of puddingstone, consisting of rounded 

 pebbles of greenstone of different varieties, of amygdaloid 

 and quartz cemented by a paste, which appears to consist 

 chiefly of trapsand united by the hard variety »f calcareous 

 spar. The paste contains also> in small quantity, zeolite* 

 prehnite, gurnet, and diailage. This puddingstone, where 

 nearest to the fort, is at least, half a mile distant from it. 

 The walls of the fort consist principally of granite, gneiss, 

 mica-slate, clay-slate, quartz, puddingstone, and pyritical 

 slate, entangled together; with a very small proportion of the 

 particular rock on which the fort itself is founded : the pud- 

 dingstone forming the greater part of them. This pudding- 

 stone Dr. M. shows to be the only verifiable ingredient of 

 The vitrificati- the walls ; and from the distance from which it must have 

 The Effect f heen brought, and the great quantity of it employed in the 

 design. work, he considers it probable, that the builders of the fort 



must have been acquainted with its verifiable nature, and 

 that it was on account of this quality, that they had em- 

 ployed so great labour in transporting it. For if their ob- 

 ject had not been to produce vitrification, but merely to 

 erect a dry wall of stone, the limestone of the hill would 

 have answered their intentions, or perhaps the loose stones 

 of the adjoining plain. That they did not obtain the 

 puddingstone from the latter source is evident ; for although 

 the plain and shore are covered with fragments, these con* 

 sist almost entirely of the primary rocks; and besides, the 

 pieces of the wall which have not felt the fire are angular 

 fragments, showing pretty clearly, that they were not col- 

 lected on an alluvial plain, but broken from the rocks where 

 they are found. Dr. iVI. next proceeds to describe the 

 various states in which the different stones are found. The 

 puddingstone exhibits the greatest variety of changes, it is 

 found in every state, from a black glass to a. spongy scoria 

 capable of floating in water, sometimes exhibiting the gra- 

 dual succession of changes from incipient calcination to 

 complete fusion. To ascertain the degree of heat necessary 

 to produce the corresponding changes in this rock, Dr. M. 

 »ubmitted various parts of it to the furnace* and he found, 

 that tome of the fused substances must have been brought 



to 



