274 1?HE CANADIAN NATURALIST; [Vol. \l 



The study of the manner and order of the formation of cry- 

 stalline minerals in coarse-grained, compound crystalline rocks, 

 has not, on the whole, had that attention which it deserves. On 

 the other hand many of the results obtained in the microscopical 

 examination of fine-grained original rocks have an important 

 bearing upon this subject. Vogelsang* has described with the 

 most painstaking accuracy his observations on the mutual rela- 

 tions of the minerals of many pitchstones, trachytes and por- 

 phyries. Mention must first be made of a very interesting phe- 

 nomenon which he has detected in the microscopical structure of 

 many trachytic and porphyritic rocks. This is called Fluidal- 

 structure, and seems to have been discovered somewhat earlier 

 and independently by E. VVeiss.f This term is to be under- 

 stood to denote such a position of the constituents of a rock re- 

 latively to each other, as to allow of the inference being drawn 

 that a movement of the mass either as a whole or in its smallest 

 parts, had taken place while the process of crystallisation or 

 solidification was going on. Eight different illustrations of this 

 phenomenon are given in the beautifully coloured plates accom- 

 panying Vogelsang's work. One of these shews a trachytic 

 pitchstone from the Eugaueau hills magnified 100 times. In a 

 brownish perfectly vitreous matrix there are found yellowish 

 grains of glassy felspar, needles of hornblende and microscopical 

 crystals of magnetite. The whole of the vitreous matrix is, be- 

 sides, filled with small prismatic crystals which are sharply 

 distinguishable from the dark ground. These, Vogelsang 

 hesitiites to declare to be felspars, and in the meantime, for con- 

 venience sake, terms them " microlites." These little crystals 

 are quite frequent in many rocks, and it is possible to distinguish 

 light and dark coloured microlites, the former being in all likeli- 

 hood scapolites or felspars, the latter augites or hornblendes. 

 The figure shews the position of these little crystals in relation 

 to the larger ones above named, and it is easily observed that 

 the former lie with their longest axes parallel to each other except 

 in the neighbourhood of the larger crystals of felspar, hornblende 

 and magnetite, around certain sides of which they crowd more 

 closely than elsewhere. The drawing shews the eff'ect of the 



* Beitriige zur Kemctuiss der Feldspath bildung, Haarlem, 1866. 

 f Vogelsang — Philosophic der Geologie nnd Microscopische Ges- 

 teins-studien'=Bonn, 1867. 



