Mr. Ewart on the Expansion of compressed elastic Fluids. 247 

 from clav-slate into hornblende-slate and greenstone is com- 

 monlv so^rradual, that we have not been able to trace anywhere 

 a line of mnction between both rocks. This transition from 

 clav-slate into the greenstone is not always formed from onestra- 

 tumtothe other; but the stratification is disturbed m theseplaces 

 bvnumberlessfissures; andthereare found masses of greenstone 

 of every imaginable contour, mingled with the clay-slate, i he 

 iron (ire) stone in the neighbourhood of Redruth and Cam- 

 borne, which is found at the surface in several places, and also 

 is well known to the miner by its extreme hardness, appears 

 to be nothing else than this kind of greenstone imbedded in 

 the killas. It is worthy of remark, that this ire-stone occurs 

 here not very far from the junction of the granite and killas 

 laid open to view in the deep mines of Dolcoath and Cook s 

 Kitchen, and that the line in which it is found differs not much 

 in its direction from that of the above-mentioned line of junc- 

 tion between the granite and killas. We may venture to say 

 that this ire-stone has not here the appearance of a regular 

 course or vein in the clay-slate, as far as our observations reach 

 on this subject. In the killas surrounding the granite of Dart- 

 moor in Devonshire, near Tavistock, also greenstone frequently 

 occurs ; it is of a very schistose kind, so that one may be in- 

 duced to consider it as clay-slate altered by some subsequent 



^"it^ would perhaps be too bold to say that all the elvan 

 courses which frequently occur in Cornwall belong to the 

 granite veins, and were going out from underlying masses of 

 granite- but we cannot forbear to remark, that some of them 

 partake very much of the appearance of the fine-grained gra- 

 nite commonly filling the granite veins, and in some places 

 more than of a porphyritic appearance ; that rocks ot a por- 

 phyritic kind are intimately connected with the granite, as at 

 Cligga Point ; that these elvan courses certainly belong to the 

 oldest formation of veins in the country, and therefore that 

 they do not differ in this respect from the granite veins. 



Swansea, Jan. 14, 1827. V. OeYNHAUSEN. V. DeCHEN. 



XXXV II I. Experiments and Ohservations on some of the Ph(S- 

 nomcna attending the sudden Expansion of compressed elastic 

 Fluid';. By Peter Ewart, Esq.* 



HAVING occasion, about seven years ago, to make some 

 experiments on a high-pressure steam-engine ot the esti- 

 mated power of nine horses, in the boiler of which the elas- 

 • 'I'his article consists of extracts from two papers read before tiie Literary 

 and Philosophical Society of Manchester ;-coini.u.nicated by the Author. 

 ^ ticity 



