52 



belong to different phases of consolidation. It does not seem probable to the 

 present writer that the views of Professor ROSEXBUSCH, so far as they affect 

 nomenclature, are likely to meet with general acceptance ; but as the argu- 

 ments advanced by him are of great importance, and incidentally involve a 

 discussion of the conditions under which different kinds of igneous rock 

 consolidate, it seems desirable that they should be considered at some 

 length. 



In the paper dealing with this subject (1) the common constituents of 

 igneous rocks are divided into four groups 



(1) The ores and accessory constituents. (Magnetite, hematite, 



ilmenite, apatite, zircon, spinelle, sphene, &c.) 



(2) The ferro-maguesian constituents. (Biotite, hornblende, pyrox- 



ene, olivine.) 



(3) The felspathic constituents. (Felspar, nepheline, leucite, melilite, 



sodalite, hauyn.) 



(4) Free silica. 



The members of the first are, according to the author, always the first to 

 form in the process of crystallisation and, as a natural consequence, they 

 show the most perfect crystalline form. Those of the second and third 

 groups differ as to their relative order of crystallisation in different cases. 

 In the granites and syenites the ferro-magnesian minerals precede the 

 felspathic constituents ; in the diabases (dolerites and gabbros) the relation is 

 frequently reversed. We have thus two types of granular structure granitisch- 

 korniy and diabasich-kornig. Where two or more members of the second 

 and third groups are present in the same rock, the order of crystallisation is 

 usually that of increasing acidity. Thus, olivine is formed before biotite, 

 pyroxene and amphibole ; biotite before either of the last-mentioned minerals. 

 Again, triclinic are usually formed before monoclinic felspars. In the 

 granular rocks of the granitic group (granitisch-koniig) , the order of 

 crystallisation and, consequently, the perfection of crystallographic form 

 correspond to decreasing basicity ; the ores and accessory constituents are 

 the first formed minerals, quartz is the youngest product in the process of 

 rock-building. The author considers that, in the entire group of the 

 holocrystalline granular rocks, after the crystallisation of the ores and 

 accessory constituents, the silicates separate in the order of decreasing 

 basicity and free silica, when present, is the last substance to crystallise. 

 Moreover, in the formation of these rocks the process of crystallisation is 

 continuous. The minerals of each of the above groups belong to one and 

 the same period ; in other words there is no recurrence of phase during the 

 process of consolidation. 



In typical porphyritic rocks the porphyritic crystals are sharply 

 differentiated from the ground-mass which may be holocrystalline, mixed 



(1) ijber das Wesen der kornigen und porphyrischen structur bei MaBserigesteinen. N.J. 

 1882, Band VI., p. 1. 



