La vis — On the Structure of Mocks. 155 



One point open to speculation is whether the presence of sodic and 

 potassic chlorides and sulphates is not the determining cause as to 

 whether the magma shall contain leucite haiiynite, nosite, or 

 socialite. For instance, we find Monte Vultura producing at 

 different epochs basalts, leucitic basalts, and haiiynite basalts, 

 which might result from the accidental introduction of such salts 

 from the sea or other sources. "We might suppose that the salts 

 are decomposed and dispersed as acids, whilst the bases are seized 

 upon by the silicic acid which, in a magma at high temperature, 

 has powerful acid properties, and so forms minerals of the leucite 

 or felspar groups. 



In this Paper I have brought together a considerable number 

 of observations, and endeavoured to glean from them the clue to 

 some of the most important problems of geological science. The 

 train of argument is somewhat disorderly; but from the large 

 number of circumstances that enter into the question of the forma- 

 tion of igneous rocks, the subject is difficult of arrangement. It is 

 unmistakably evident that if the young science of petrology is 

 intended to be carried beyond the simple dry description of rock 

 masses, it must be brought to bear upon the various modifications 

 and derivatives of them, in any given district, and also that it will 

 never supersede field investigation ; but by the two going hand-in - 

 hand they may open the doors and show us the secrets of Nature's 

 great chemical laboratory — our globe. 



