222 OUTLINES OF FIELD-GEOLO PART n 



geologist's accoutrements an acid - bottle, or some 

 powdered citric acid to be used with a drop of 

 was included (ante, p. 30). A little acid will at on' 

 by a brisk effervescence if a rock is a In IK -stone or is 

 markedly calcareous, liy the saim- i.. may often 



trace the decomposition of such rocks as dolerite to a 

 considerable distance inward from the surface ; the 

 original lime-hearing silicate of the rock having been 

 decomposed by infiltrating rain-water, and partially 

 converted into carbonate of lime. This carbonate is 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 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. By this means a 

 copious discharge of carbonic acid may be obtained from 

 some rocks where otherwise it might appear so feebly as 

 perhaps even to escape observation. Some carbonates, 

 dolomite for example, are hardly affected by acid until 

 heated. In other cases, the acid requires to 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 

 weak hydrochloric acid. If there is much impurity (clay, 

 sand, oxide of iron, etc.), this will remain behind as an in- 

 soluble 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 every- 



