56 



in Cab. Cyclop., vol. ii., p. 98). Much doubt, however, still hangs 

 about this subject. Cases occur in which magnesia has been intro- 

 duced, although the limestone could not have been subject to such 

 a pressure as would confine its carbonic acid when the rock was 

 softened by heat. 



In order to elucidate, if possible, this obscure subject, two speci- 

 mens of the rock were submitted to Mr. John Macadam, lecturer 

 on chemistry, now of the Melbourne Philosophical Institution, for 

 examination with reference to the presence or absence of magnesia. 

 The following is Mr. Macadam's report ; the specimen referred to 

 as No. 1 is the saccharine marble from contact with the dike ; No. 2 

 is the unaltered limestone both were average specimens : 



" I have carefully subjected to chemical analysis the specimen of 

 limestone No. 1, with special reference to the presence or absence 

 of magnesia ; and I find from the indications given that carbonate 

 of magnesia constitutes about 2^ per cent, of the whole mass. The 

 mineral is not, therefore, a double carbonate of lime and magnesia. 

 Its other and principal ingredients are carbonic acid and lime, 

 besides which silica is present, as also traces of oxide of iron and 

 alumina. 



" In the specimen No. 2, I find magnesia in great abundance, the 

 amount present being equivalent to 33*72 per cent, of carbonate of 

 magnesia. The other constituents present are similar to those 

 reported in No. 1. From the large proportions of carbonate of lime 

 and carbonate of magnesia present in specimen No. 2, it would 

 appear to be a species of dolomite. It may be noticed that the 

 physical characters of No. 2 are very different from those of No. 1 ; 

 the former is difficult to pulverize, the latter is extremely susceptible 

 of division. 



" The action of strong hydrochloric acid on both specimens causes 

 a portion of gelatinous silica to appear, showing the presence of a 

 silicate, which may be that of magnesia, since the quantity of gelati- 

 nous silica is about sufficient to combine with the 1*28 per cent, of 

 caustic magnesia existing in the specimen No. 1. There is a less 

 quantity of this gelatinous sih'ca in No. 2. The greater portion, 

 however, of the silica present in both specimens remains undissolved 

 in the gritty or pulverulent condition, and is hence in a state of 

 mere mechanical mixture with the other constituents of the lime- 

 stone. It would require a minute quantitative analysis to determine 

 whether the T28 per cent, of magnesia exists as a carbonate or 

 silicate, or partly as both." 



The phenomena are thus of a contrary character to what is usually 



