152 MINERALS AND GEOLOGY 



ing vegetable matter, &c.) dissolves a certain amount of carbonate 

 of lime, but the bicarbonate, thus formed, is easily decomposed by 

 various natural agencies, even by mere exposure to the atmosphere, 

 and a precipitation of calcareous matter takes place. In this manner 

 calcareous tufas (so common in many of our swamps, streams, &c.),. 

 together with stalactites and stalagmites, are produced ; and similar 

 processes, acting on a larger scale, may have given rise to extensive 

 depositions of limestone strata in ancient seas and lakes. Some lime- 

 stones, again, are formed almost wholly of the calcareous shells or 

 tests of crinoids, foraminifera, and other organisms (see Part IV) : 

 but others are, undoubtedly, mechanical or rock deposits, derived 

 from the wasting of coral reefs and other limestone formations. 

 Limestones consist of carbonate of lime, more or less pure ; dolomites, 

 of carbonate of lime and carbonate of magnesia in equal atomic pro- 

 portions ; and dolomitic limestones of these two carbonates in other 

 proportions, the lime carbonate generally predominating. Dolomites- 

 and dolomitic limestones appear in many cases to have been simple 

 chemical precipitates, and, in others, to have originited from the- 

 alteration of limestone rocks by the action of soluble magnesian salts. 

 These calcareous rocks are of various colours : grey, white, black, 

 yellowish, &c. Their texture is sometimes very close and uniform. 

 At other times, the stone is made up of small spherical concretions, 

 when the texture is said to " oolitic." A bed of grey limestone of this 

 structure occurs near the Chatte Biver in Gaspd Oolitic limestones 

 are of all geological ages. Some limestones, again, are of an earthy 

 texture : the well-known chalk of Europe is an example ; also our 

 own "calcareous tufa," or " shell marl." Many of the dark lime- 

 stones, as those of Niagara, &c., are more or less bituminous. Ordi- 

 eary limestones dissolve in acids with strong effervescence ; but dolo- 

 mites as a rule produce merely a feeble or slightly perceptible effer- 

 vescence unless the acid be heated.* Limestones which contain from 

 15 to 25 per cent, of argillaceous matter in intimate admixture, yield 



*~To determine the presence of magnesia in dolomitic limestones, a few grains of the rock 

 may be dissolved in dilute hydrochloric acid. The solution is then boiled with a drop or two 

 of nitric acid (to convert any FeO, that may be present, into Fe 2 O 3 ), and ammonia is added 

 carefully in slight excess. This will occasion a floculent precipitate if iron be present. Oxalate 

 of ammonia is then added to precipitate the lime ; this (after settling) is filtered off ; the filtrate- 

 tested with another drop of oxalate of ammonia to make sure that all the lime has been thrown 

 down ; and finally the magnesia is precipitated by sodium phosphate or by solution of the 

 blowpipe flux known as " microcosmic or phosphor salt." 



