I 1 J. P. IDDINGS 



brought about. This may reveal parts of all the rock-bodies 

 originally present, or much more probably only a certain number 

 of all once in existence, some having been entirely removed or 

 still remaining covered. The rocks exposed in any region 

 seldom represent the whole series of varieties that actually 

 exist or did exist in the region. Consequently some series or 

 associations represent comparatively few varieties of rocks, and 

 these often quite different from one another, as when only 

 basalt, rhyolite, and one or two varieties of andesite occur in 

 a region, whereas other series exhibit many varieties and fre- 

 quent transitions from one to another as in the dissected vol- 

 canoes in the Yellowstone Park region. Again, it is found that 

 the range of rock varieties in some regions is limited, and in 

 others is very wide, indicating less differentiation of the parent 

 magma in one case than in another. 



A knowledge of these associations in various regions leads 

 to a comprehension of the laws governing the production of 

 varieties of igneous rocks, both their probable differentiation 

 from a parent magma, and something of the mechanism of their 

 eruption, consequently its importance in a treatise on petrology 

 cannot be overestimated. The consideration of these relation- 

 ships is absolutely essential to a right conception of the true 

 nature of igneous rocks. 



Classification of igneous rocks and the didactic treatment of petrol- 

 ogy. — It is hoped that the foregoing discussion has made it 

 apparent that a systematic classification of all kinds of igneous 

 rocks cannot be put on the same basis as a philosophical treat- 

 ment of the subject-matter of petrology, which takes cognizance 

 not only of the material character of rocks, but also of the laws 

 governing their production, eruption, mode of occurrence, and 

 solidification, as well as their subsequent alteration. 



The object of a classification of rocks is clearly the bringing 

 together of those that have like characters in order that they 

 may receive a common name, and that their descriptions may be 

 systematically arranged for convenient reference. The use of 

 names common to all similar rocks instead of individual names, 

 such as are given to men, is also solely for convenience ; it being 



