applied to the study uf some Metamorphic Rocks. 171 



represented by the following numbers : — 



Carbonate of lime 57-483 



Carbonate of magnesia 40*974 



Sesquioxide of iron and alumina . . 0*411 

 Water and organic matter . . . . 0*239 



Silica 1-095 



100-202 



From this analysis it would appear that the mineral to which 

 I have referred is a dolomitic rock ; but it affords us no informa- 

 tion whatever relative to its real nature or origin. If, however, 

 instead of operating on the mineral in the form of powder, or 

 fragments coai'sely broken up, we proceed by means of diluted 

 hydrochloi-ic acid acting on a single fragment of moderate dimen- 

 sions cut in the direction of the fibres, we shall observe, after 

 continuing the process for some days, that there will be left an 

 asbestiform skeleton having the following composition : — 



Silica 68 



Magnesia*. ... 29 



— numbers representing a magnesian tremolite. It is from this 

 simple difference in the manner of conducting the experiment 

 that a result so different from the former has been arrived at, 

 and one that enables us to trace, so to speak, the real origin of 

 the rock in question. 



Some varieties of magnesite (siliceous carbonate of magnesia) 

 similarly treated with dilute hydrochloric acid, leave silico-gela- 

 tinous residues, which afford indications, as in the former case, 

 of the origin of the rock from which they are derived. Another 

 case in point is the possibility of following the transition from 

 meerschaum, which is a definite silicate of magnesia, to a repla- 

 cing pseudomorphite of ordinary carbonate of magnesia contain- 

 ing mei-e traces of silicates. 



Another example is afforded by the manner in which concen- 

 trated hydrochloric acid acts upon crystals of certain varieties of 

 zeolites with alkaline bases. This action shows, from the partial 

 decomposition effected, the stages of alteration through which 

 these minerals pass. A crystal of Thomsonite boiled with hy- 

 drochloric acid, deposits, after the saturation of the alkali, a ge- 

 latinous transparent precipitate of silica ; and an opaline skeleton 

 remains, which, to a certain extent, presents the outlines of the 

 primitive crystal. A mass of these crystals treated in the same 

 manner, and dried after separation of the gelatinous silica, resem- 

 bles in appearance some of the siliceous aggregates which are 

 often found in solfataras and in other volcanic localities as well 



* Vide Phil. Mag. for March 1858, 



N 2 



