ANKE VERS ART ADDRESS OF THE PRESIDENT. 75 



bution of miuerals in space and time, and the^mannsr in which they 

 are associated with one another to build up rock-masses, constitutes 

 a most important branch of our science, that to which the name of 

 Petrology is given. 



Under the name of " Petrography " an attempt has been made to 

 establish a branch of Xatural-History science which shall bear the 

 same relation to Mineralogy as that science does to Chemistry. As 

 minerals are formed by the union of certain chemical compounds, so 

 rocks, it is argued, may be regarded as being built up of different 

 minerals. But it must be remembered that while minerals possess 

 a distinct individuality — the result of their different chemical con- 

 stitution and their characteristic crystallographic form — we are 

 quite unable to poiut to anything analogous to these in the case of 

 rocks. 



How is a rock- " species " to be defined ? It is not enough to state 

 its ultimate chemical composition ; for rocks of the most varied 

 character and origin may agree in this respect. Equally futile 

 is it to take miueralogical constitution as the basis of our classifi- 

 cation ; for, in the same rock-mass, the species of minerals which 

 are present and their proportions to one another may, and, indeed, 

 often do, vary from point to point, ^or does minute structure, 

 though affording admirable criteria for distinguishing certain types of 

 rock, supply a sufficiently definite means of diagnosis for all the 

 different varieties which occur. A system of "lithology" may, 

 indeed, be devised, if we confine our attention to the hand-specimens 

 in our museums ; but it breaks down the moment that we attempt 

 to apply it in our researches in the field. 



I have long felt assured that all attempts at a nomenclature and 

 classification of rocks must, for the reasons just stated, be regarded 

 as tentative and provisional only ; but the careful study of rocks is 

 nevertheless bringing to light a number of facts calculated to pro- 

 foundly modify miueralogical no less than geological thought and 

 speculation. 



Petrology forms the link between Mineralogy and Geology, just as 

 Pala3ontology does between Biology and Geology. Mineralogy has 

 justly been styled the alphabet of Petrology ; but if the orthography 

 and etymology of the language of rocks lie in the province of the 

 Mineralogist, its syntax and prosody belong to the realm of the 

 Geologist. In that language, of which the letters are mineral 

 species and the words are rock- types, I am persuaded that there is 

 written for us the whole story of terrestrial evolution. 



