348 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



were made to separate from them any mineral elements which 

 were foreign to the rock, or had come into the magma before 

 it solidified, and also all secondary elements which had formed 

 after the consolidation of the rock during the processes of 

 internal decomposition or interaction. 



Excellent work has been done in this field of research by 

 Roth, Bischof, Delesse, Zirkel, Broegger, and Iddings. 



Certain principles are usually inculcated regarding the 

 sequence in which the minerals take origin during the passage 

 of a magma from the viscous to the solid state, but the prin- 

 ciples are by no means always applicable, and have therefore 

 frequently been contested. Minerals which have crystallised 

 with the most complete and perfect form have usually been 

 regarded as the first-formed, while those which appear to 

 have been checked in their proper development by others, 

 have been regarded as of later formation. Again, minerals 

 that are enclosed within other minerals are usually taken to 

 be older than the enveloping material, yet cases are cited 

 where they are really younger, having separated out from a 

 portion of the magma enclosed within the developing mineral. 

 Minerals without any inclusions for the most part belong to 

 the first generation of solid material. If two minerals occur 

 as intergrowths with one another, contemporaneous generation 

 is indicated. In rocks with porphyritic structure the larger 

 mineral forms are as a rule older than the ground-mass. 



It was in accordance with these principles that Fouque 

 and Michel-Levy first distinguished different generations of 

 minerals, and used the number of the mineral generations as a 

 distinguishing feature between rocks of granitic and porphyritic 

 structure. Through a large number of individual observations 

 it -has been possible to determine genetic series for the rock- 

 forming minerals. Certain minerals, such as magnetite, 

 titanite, rutile, apatite, zircon, spinel, olivine, belong generally 

 to the earliest products of separation, preceding the augites, 

 hornblendes, felspars, and quartz. Rosenbusch holds the 

 opinion that in the deep-seated rocks, at any one interval of 

 time, there is only one kind of mineral separated from the 

 magma. The periods of formation for the different constituents 

 succeed each other so that either those of one kind do not form 

 until the complete separation of the preceding kind; or much 

 more frequently, a younger constituent in order of separa- 

 tion begins to form a certain time before the completion of the 



