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 River in Gaspe. 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 t wo- 

 of nitric acid (to convert any FeO, that may be present, into Fe 2 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." 



