¥, es M4 ; is ae > no Ri a cee SA we ps 
= - ; _ . oe = COR ee ao ee ee Ms 

192°" “GHOGNOSY. fo [Boox IT. | 
stone and markedly calcareous rocks may thus at ee detected. | 
By the same means the decomposition of such rocks as dolerite may 
be traced to a considerable distance inward from the surface; the ~ 
_ original lime-bearing silicate of the rock having been decomposed by 
infiltrating rain water, and partially converted into carbonate of lime, 
This carbonate being far more sensitive to the acid test than the 
other carbonates usually to be met with among rocks, a drop of weak 
cold acid suffices to produce abundant effervescence even from a 
crystalline face. But the effervescence becomes much more marked 
if we apply the acid to the powder of the stone. For this purpose a 
scratch may be made and then touched with acid, when a copious — 
discharge of carbonic acid may be obtained where otherwise it might 
appear so feebly as perhaps even to escape observation. Some car- 
bonates, dolomite for example, are hardly affected by acid until 
powdered. In other cases the acid requires to be heated, or must be 
used very strong, as with siderite. 
It is a convenient method of roughly estimating the purity of a 
limestone to place a fragment of the rock in hydrochloric acid. 
If there is much impurity (clay, sand, oxide of iron, &c.), this will 
remain behind as an insoluble residue, and may then be further 
tested chemically or examined with the microscope. Of course the 
acid may attack some of the impurities, so that it cannot be concluded 
that the residue absolutely represents everything present in the rock 
except the carbonate of lime, but the proportion of non-calcareous 
matter so dissolved by the acid will usually be small. 
Hydrofluoric acid isa reagent of considerable service in separating 
the mineral constituents of rocks. The rock to be studied is reduced 
to powder and introduced gently into a platinum capsule containing 
the concentrated acid. During the consequent effervesence the 
mixture is cautiously stirred witha platinum spatula. Some minerals 
are converted into fluorides, others into fluosilicates, while some, 
particularly the iron-magnesia species, remain undissolved. The thick 
jelly of silica and alumina is removed with water, and the crystalline 
minerals lying at the bottom can then be dried and examined. By 
arresting the solution at different stages the different minerals may 
be isolated. ‘This process is admirably adapted for collecting the 
pyroxene of pyroxenic rocks." 
3. Further chemical processes—A thorough chemical analysis of 
a rock or mineral is indispensable for the elucidation of its com- 
position. But there are several processes by which, until that 
complete analysis has been made, the geologist may add to his 
knowledge of the chemical nature of the objects of his study. It is 
commonly the case that minerals about which he may be doubtful 
are precisely those which, from their small size, are most difficult 
of separation from the rest of the rock preparatory to analytical pro- 
cesses. ‘The mineral apatite, for example, occurs in minute hexa- 
gonal prisms which on cross-fracture might be mistaken for nepheline, 
* Fouqué et Michel-Lévy, op. cit, p. 116. 

