IxXXVi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9OI, 



That it is a vera causa is admitted on all hands, but differences of 

 opinion exist as to the extent to which it should be applied in 

 explaining the origin of petrographical species. 



If we study igneous rocks which have appeared at the surface as 

 lavas, or have been intruded at moderate depths as dykes, sills, 

 laccolites, or bosses, the evidence of absorption is, in my judgment, 

 so slight as to be practically negligible ; but if we pass from such 

 regions to others in which plutonic rocks are found in relation with 

 crystalline schists and study ' les appareils granitiques a racines 

 profondes ' of M. Michel Levy, the case is different. It may be 

 that the final solution of the problem of the origin of igneous 

 magmas will be found in these regions ; but here we touch a 

 question which belongs to the future rather than to the past, and 

 lies, therefore, beyond the scope of this Address. So far as I am 

 concerned, I will confess that my ideas are not fixed. At present 

 I am not disposed to attach much importance to theories involving 

 differentiation in situ by unaided molecular fiow in dykes and 

 laccolites ; but rather to attribute such variation as does occur to 

 successive eruptions, or to a continuous change in the nature of 

 the material during the process of intrusion. The great difficulty 

 in applying any theory that involves differentiation in situ to such 

 cases arises from the slight effect of the igneous magmas on the 

 containing walls — a fact which negatives the idea that the material 

 arrived at the place where we now find it in a condition of super- 

 fusion, or that it remained fluid long enough to enable any con- 

 siderable diffusion to take place. 



Our ideas as to the origin of igneous rocks are still ' en pleine 

 evolution.' Conditions are rapidly changing in consequence of dis- 

 coveries in geology and physical chemistry. Eival theories are 

 struggling for existence, and although it is safe to predict that some 

 will become extinct, that others will be modified, and that natural 

 selection will finally bring about the survival of the fittest, it is 

 impossible to determine, at present, the relative importance of those 

 which claim our attention. 



The origin of petrographical species, so far as the igneous rocks 

 are concerned, is a problem the final solution of which has been 

 handed on by the nineteenth century to its successor. 



