1 8 Igneous Rocks. 



calcareous, large percentages of bicarbonate of lime in 

 solution. The reason of this is, that all rain in descend- 

 ing through the air takes up a certain amount of 

 carbonic acid one of the constituents, accidental or 

 otherwise, of the air ; and this carbonic acid has the 

 power of dissolving the carbonate of lime which enters 

 into the composition of a large proportion of stratified 

 rocks, which sometimes as pure limestone, form great 

 tracts of country. In this way it happens that springs 

 are often charged with lime, in the form of a soluble 

 bicarbonate, which is carried by rivers into lakes and 

 estuaries, and, finding its way to the sea, affords 

 material to shell-fish and other marine animals, through 

 their nutriment, to make their shells and bones. Thus 

 it happens that, by little and little, lime is abstracted 

 from sea-water to form parts of animals, which, dying 

 in deep clear water, frequently produce by their skele- 

 tons and shells immense masses of strata of nearly pure 

 limestone, which is consolidated into rock almost as 

 fast as it is formed. 



What is going on now has been going on throughout 

 all known geological time, from that of the deposition 

 of the Laurentian rocks down to the present day. 



Igneous rocks form a much smaller proportion of 

 the surface rocks of most parts of the world, though in 

 given areas, such as Iceland and the Faroe Islands, they 

 largely predominate. To take Britain as an example : 

 in North Wales, a considerable proportion, perhaps a 

 twentieth part, of the rocks of Lower Silurian age are 

 formed of igneous masses. The whole of the rest of 

 Wales, till we come to Pembrokeshire, contains almost 

 none whatever. In Cumberland a very large part of 

 the Lower Silurian rocks are igneous, while a com- 

 paratively small proportion of igneous rocks is found 



