NATUKE OF CHALK. 201 



of an exceedingly fine-grained homogeneous soft substance, 

 adhering strongly when applied to the tongue, and leaving 

 a white streak when rubbed on other substances. If 

 treated with vinegar, or any other acid, it will effervesce 

 strongly with the liberation of the gas commonly known 

 as choke-damp, or carbonic acid, while the base unites with 

 the new acid to form a fresh compound of lime. The 

 lime may be obtained in a pure condition by burning the 

 chalk, as in a lime-kiln, when the carbonic acid is likewise 

 given off ; and we thus learn that chalk consists of 

 carbonate of lime. As a rule, when we examine a chalk- 

 cliff we shall find that the chalk, although stained here 

 and there with iron, is identical in structure throughout 

 great thicknesses, and that it shows nowhere any signs of 

 crystallization. Occasionally, however, as at Corfe Castle, 

 near Swanage, in Dorsetshire, we shall find that the chalk 

 has become so hard as to leave no distinct streak when 

 rubbed lightly on other substances ; while its cracks and 

 fissures are filled with translucent crystals of white spar 

 the calc-spar, or calcite of mineralogists. Here, then, we 

 have the chalk so hardened, probably by the effects of 

 subterranean heat, as to form what is popularly called a 

 limestone ; while a farther step would have converted it 

 into actual marble. The geologist would indeed apply 

 the name limestone to chalk, ordinary limestone, and 

 marble indifferently ; but since the popular usage is 

 different, it is well to be assured that all three are but 

 various modifications of one and the same substance. In 

 the north of Ireland the basalt of the Giant's Causeway 

 has converted the chalk still more completely into a hard 

 limestone. 



Chalk, then, may be defined as a fine-grained, white, 

 non-crystalline, soft limestone. This, however, by no 

 means exhausts the subject of its composition. Thus 

 if we take a piece of chalk and wash it carefully in water 

 with a hard brush so as to reduce it to a state of mud, 

 and examine the portion which falls to the bottom of the 

 vessel under a microscope, we shall find that this is very 

 largely made up of various shell-like substances. Many 

 of these are minute fragments of \vhat may have been real 



