A STUDY IN CONSANGUINITY OF ERUPTIVE 



ROCKS. 



Without being distinctly formulated, the principle of con- 

 sanguinity recently enunciated by Prof. Iddings has, as a working 

 hypothesis, been the guide of studies made within the last few 

 years on a group of Brazilian eruptive rocks, and the means of 

 arriving at some interesting and, in part, novel results. The 

 method of study followed, partly by plan, partly from force of 

 circumstances, being the comparative study of a group of locali- 

 ties on the assumption of genetic relations between them, rather 

 than detailed work at single points, was similar to what would 

 be applied to the study of a sedimentary group. This method 

 has in this case proved of great advantage, and, as a contribution 

 to the subject of consanguinity, seems worthy of being put on 

 record. 



In 1883, the writer, whose previous training had been almost 

 exclusively in the domains of palaeontology and the distinctly 

 sedimentary formations, finding himself in a region of crystalline 

 and metamorphic rocks felt the need of acquainting himself with 

 modern petrographic methods. Working in complete isolation 

 without previous instruction in this branch, without material for 

 comparison and almost without literature, he was also without 

 the traditions of the science and preconceived ideas of the 

 relations of the different petrographic groups, and thus free to 

 follow out the lines of investigation suggested by their apparent 

 field relations. 



In working over the material at hand in the National 

 Museum at Rio, attention was attracted to specimens of nephe- 

 line-syenite, or foyaite (using that term as a general title for the 

 holocrystalline nepheline-orthoclase rocks) and as one of the 

 localities, the peak of Tingua, was readily accessible from Rio 

 an attempt to determine its field relations was resolved upon. 

 This heavily wooded mountain proved a hard nut to crack, and 

 several excursions gave very slender results beyond the fact that 



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