h THE ISUILniNG OK AN ISLAND. 



and clay and soft marls arc included among rocks, as well as limestones, grit- 

 stones or granites. It is in this more general sense that the word is used 

 in these cliapters. 



The Two Rock-Fokmations of the Island. 



We find then in the island two great classes of rocks, those which belong 

 to the '' bliie-bcack" set, and those which belong to the marls and limestones. 



That there is some essential difference between these two classes of rocks 

 may be shewn in a very simple wav^ We take two glasses, each containing a 

 little diluted muriatic or other acid, and into one of them we drop a "blue- 

 beach " pebble, while into the other we drop a fragment of limestone, and we 

 see that the limestone immediately begins to effervesce while the blue-beach 

 remains nearly or entirely unmoved. What is the reason for this singular be- 

 haviour on the part of the limestone? It is that the stone is for the most part 

 a combination of lime with carbonic acid gas and that the latter is released by 

 the action of the aciil and comes awav in bubbles. The blue-beach pebble 

 behaves differently because it is (|uite a difTerent substance, being for the 

 greater part a combination of the substances which make clay and quartz. 



It is plain then that we have two great classes of rocks in the island, and 

 we shall see later that, although the histories of these two great divisions are 

 in some respects alike, their modes of origin are for the most part very 

 different. 



