76 report— 1859. 



skeleton of a former rock. In the generality of instances silica is dissemi- 

 nated through the mass, and then does not form a coherent skeleton, so that 

 the garnets, instead of being surrounded with the amorphous silica, are only 

 coated on some sides with a siliceous film. 



Cases also occur in which there is no silica, and the limestone contains 

 only a small quantity of the above mentioned green mineral. In fact every 

 analysis of the rock will give a different result as regards the quantity and 

 nature of the residue. 



The analysisof the green mineral was attended with great difficulty in conse- 

 quence of the fine state of division in which it occurred, besides being mingled 

 with idocrase and garnet debris nearly as fine as itself. A small quantity of 

 it, picked out with the aid of a lens, yielded the following results: — 



Lime 26*183 



Magnesia 8*825 



Protoxide of iron 10*576 



Alumina 3*750 



Silica 49-6*1 



Water 1*025 



100*000 

 These numbers show that it is an aluminous augite, or more properly an 

 augite and garnet compound, corresponding in a most striking manner with 

 one from Fassa analysed by Kudernatsch* ; the sum of the oxygen in the 

 protoxide bases being 13*360 for the green mineral, and 13*658 for the spe- 

 cimen from Fassa. 



As broken fragments of minerals cannot have been formed by the action 

 of heat upon a sedimentary limestone, the fragmentary state of the green 

 augite, idocrase, and garnets prove that the rock under discussion has not 

 been metamorphosed by heat. On the other hand, the existence of an enve- 

 lope of soluble silica surrounding some of the garnets, seems to show that 

 anterior to the formation of the new limestone rock, one existing of silicates 

 must have existed there. 



Experime,nts to determine the Efficiency of Continuous and Self-acting 

 Breaks for RailwayTrains. By William Fairbairn, F.R.S. 



Of late years various improvements have been introduced upon railways to 

 diminish the dangers of travelling, and attention is now specially directed to 

 the increase of the retarding power for trains by various kinds of breaks. 

 From an early period in the history of railways it was seen that few objects 

 were more important for ensuring the security of passengers and reducing 

 the loss of time occasioned by stoppages, than the attainment of some means 

 of destroying the momentum of trains with ease and rapidity, that is, in the 

 least time and in the shortest distance. The less the time requisite to break 

 a train, the longer the steam may be kept on in approaching a station, and 

 the less is the loss of time in stopping; and the shorter the distance in which 

 a train can be brought to a stand, the less danger is there of collision with 

 obstructions on the line perceived not far off ahead. It is already allowed 

 by many of those connected with railways, and has been expressly stated by 

 the Lords of the Committee of Privy Council for Trade, that the amount of 



* Gmelin, Handbuch d. Chemie, Bd. 2. p. 383. 4 Auf. 



