'266 L. V. Pirsson — Classification of Igneous Rocks. 



in intrusive occurrences we find laccoliths grading into 

 intrusive sheets, stocks running out in dikes, etc. This 

 is true not only of the individual bodies, but of the 

 masses grouped collectively; thus we may find occur- 

 rences in which it is arbitrary whether we shall call them 

 intrusive sheets or laccoliths, dikes or stocks, stocks or 

 batholiths. And so petrographically we may find one 

 rock mass, like a stock, composed of different kinds of 

 material sharply separated from one another, whereas 

 in another mass one variety of rock substance passes 

 most gradually into another variety, while collectively 

 if the materials composing all igneous rock bodies are 

 compared there will be found, mineralogically, and there- 

 fore chemically, whole series giving complete transitions 

 from one kind into another. 



It is therefore clear that any system of classification 

 of the igneous rocks must be an arbitrary one, and an 

 examination of those that have hitherto been attempted 

 will prove the truth of this statement. Some which have 

 been termed natural by their authors will be found to have 

 divisions that seem natural only to the authors who 

 made them. Others see no* natural divisions in these 

 places but would draw distinctions elsewhere. This is 

 inevitable when one perceives that they are matters of 

 opinion, and history teaches us that in matters of opinion 

 men have always differed, and from this we infer that 

 they are likely to continue to differ. The situation is 

 similar to that of any continuous field .which requires to 

 be divided, like that of temperature or length for exam- 

 ple, in which we see that one nation uses one scale and 

 another nation uses a different scale. It will be profitable 

 for us to consider briefly some of the more important 

 attempts at classification and the principles upon which 

 they are grounded. 



Some Systems of Classification. — ^At the outset we must 

 observe that it is possible to regard and classify igneous 

 rocks from two different view-points. In one we may 

 take account of the form, disposition, and relation of an 

 igneous rock-mass and its relation to other rock-masses, 

 in short its nature as a component unit in the architecture 

 of the earth's outer shell. This is its mode of occur- 

 rence, whether intrusive or extrusive, whether a flow, 

 dike, stock, etc., and is a, geologic way of regarding it. 



