432 Re'pprts and Proceedings. 



naked eye, hence the name Aphanite given by Hauy. The minute- 

 ness and intimate union of the ingredients of Aphanite when quite 

 compact make it impossible with the ordinary aids to discover 

 whether it belongs to Diabase, Gabbro, or Diorite." Now it appears 

 to me that to call a rock Aphanite, or to say that a classification of 

 Greenstones is impossible, is just as absurd as it would be for a 

 Zoologist or Botanist to assert that no satisfactory account could be 

 given of animal or vegetable tissues because they cannot be made 

 out by the naked eye or a pocket lens. The examination of many 

 rocks, if not too fine grained, may be facilitated by examining a 

 polished surface with a low power, or even by simply wetting the 

 freshly -broken surface instead of polishing ; but in the case of 

 basaltic and other fine-grained rocks it is necessary to prepare thin 

 sections for examination by transmitted light. This method is, in 

 fact, by far the most satisfactory in the majority of cases, for not 

 only may the various constituent minerals be readily observed, but 

 the order in which they crystallized out from the mass, the changes 

 which they have frequently undergone during the long lapse of ages, 

 and other important facts, may also -be thus ascertained. 



A microscopical examination of rocks shows that, as a rule, the 

 igneous rocks may be distinguished at once from all others by their 

 structure, which is that of a more or less perfect network of miuute 

 crystals : in many cases all the minerals are well crystallized ; in 

 others there is an amorphous or glassy base in which they are 

 enclosed ; there are, however, other rocks, such as the Felstones and 

 the more recent volcanic Phonolites, some of which do not present 

 this crystallized arrangement of their constituents.; and then there 

 are the Porphyrites, which are characterized by the presence of 

 crystals of felspar ia a compact felspathic base. There is in fact a 

 gradual passage from the compact Felstones to the Porphyrites, so 

 that it appears probable that the amorphous base of all such rocks is 

 simply the siliceous magma or paste from which distinct minerals 

 would have separated had the circumstances under which they were 

 formed been favourable for crystallization to have taken place. As 

 a typical example of a widely distributed class of rocks, I will take 

 for description the well-known basaltic rock of the Eowley Hills. 

 An examination of thin sections shows that it contains a triclinic 

 felspar, augite, magnetic oxide of iron, a little olivine, and a few 

 crystals of apatite. The felspar is known to belong to the triclinic 

 system, as it exhibits the characteristic strise when examined by 

 polarized light. Augite occurs in minute black shining crystals, 

 which appear bright brown, or occasionally green when in thin 

 sections : it cannot be mistaken for any other mineral except horn- 

 blende, from which it is distinguished by a marked difference in the 

 angles ; it has also a clear glassy appearance, while hornblende is 

 either distinctly fibrous in texture, or exhibits lines or cracks running 

 parallel with the principal cleavage plane. Very frequently horn- 

 blende is green, while augite is yellowish brown, but this does not 

 always hold good. The magnetite occurs in minute grains, or as 

 thin lamina, both being black and opaque. The apatite is seen in 



