212 PROCEEDINGS OF THE AMERICAN ACADEMY. 



eruptive rocks have been, for a generation, the usual guide to the many- 

 authors who have described their findings among the igneous terranes 

 of the world. 



In view of these facts it is clear that a student in petrology who 

 wishes to use the maps and memoirs should have a good conception of 

 the rock-types recognized by Rosenbusch and by his hundreds of dis- 

 ciples among the field-geologists. It is true that in some details the 

 usages of master and followers as regards names and classification have 

 varied, but in a broad way Rosenbusch's definitions of the principal 

 families and species of massive rocks have been used for maps and 

 reports in all regions where modern work on igneous geology has been 

 done. Just as the general sequence of the stratified rocks as first de- 

 scribed in England, France, and Germany has been found to be closely 

 paralleled in the rest of Europe and in the other continents, so the 

 system of igneous rocks as at first developed from material largely 

 collected in Europe has been nearly sufficient for the mapping of those 

 rocks elsewhere. In the field as in the library the geologist soon 

 learns that there is a persistent recurrence of types in the larger divi- 

 sions of the earth's surface. The usefulness and objective character 

 of Rosenbusch's classification are, therefore, proved by its adaptability 

 in all the continents and islands. 



Rosenbusch and his followers recognize some latitude of variation in 

 the composition of each rock-type. The variation is both mineralog- 

 ical and chemical, two rock specimens referred to a type showing 

 differences in the proportions of the chemical elements found by analy- 

 sis of the two rocks. In fact, no two analyses of granite, andesite, or 

 any other one type have ever given precisely the same proportions of 

 the dozen or more oxides which regularly make up an igneous rock. 

 It is obvious that the student of map and memoir should, for many 

 problems, have at hand the actual figures showing the most typical 

 chemical composition of the rock-types to which his study is directed. 

 In numerous cases an analysis of a single specimen is not so useful as 

 that which could be made from a thorough mixture of specimens of the 

 same rock- variety from all places on the globe where that variety 

 occurs. 



For obvious reasons such ideal analyses have never been made. In 

 their stead the writer believes that the investigator of petrogenic and 

 other world-problems may well use the averages calculated from the 

 many excellent chemical analyses of rocks made since Rosenbusch's 

 system of naming and classification has been in general use. It may, 

 indeed, be argued that such averages would more nearly represent the 

 chemistry of Rosenbusch's types than any of the respective single 



