84 PEOCEEEZXGS OF THE GEOLOGICAL SOCLETr. 



composition of the rock. Some slides, however, illustrate another 

 form of disturbance of equilibrium : crystals of quartz or felspar are 

 imbedded in the rock, and on these the larger belonites have, as it 

 were, grown, in accordance with a well-known law of crystal- 

 building, that a mechanical disturbance of equilibrium is favourable 

 to it, and every crystallite seeks its wov oTio. 



Let us now proceed to the igneous rocks which are comprehended 

 in the general term '"' trachytes ;" rocks which still retain a glassy 

 base, but have it crowded with microliths ; in which also there is 

 commonly a slightly lower percentage of silica than in such glassy 

 forms as pitchstone and obsidian. Here we find crystallites of 

 felspar largely developed, together with granules of augite, horn- 

 blende, and magnetite ; these occasionally are so far associated as to 

 afford instances of twinning, and they vary notably in size. In these 

 rocks we are presented with a stage of crystal-building somewhat 

 analogous to that just described, though taking place in a rock of 

 slightly different composition. This microlithic structure appears ta 

 me to indicate that temperature (as in the former case) was changing 

 rather rapidly, and crystal-building was arrested before it had 

 progressed beyond one or two of its earlier stages. Hence I fully 

 expect that it will be restricted to rocks which have either been 

 emitted as lavas or, if intrusive, have solidified not far from th& 

 surface of the ground, that is, under circumstances which have 

 allowed of comparatively rapid cooling and perhaps the free evapora- 

 tion of water. 



Let us now for a moment turn our attention to the larger crystals 

 of quartz, felspar (and, in some more basic examples, nepheline and 

 leucite), biotite, augite, and hornblende, which we find in porphyritic 

 varieties of these rocks. These are not seldom found to have incor- 

 porated into themselves portions of the ground-mass or microliths 

 of other minerals such as occur in the rock, which have been forced 

 to obey the law of crystallization of their captor, and to arrange 

 themselves conformably to it. Instances are too common to require 

 enumeration. I interpret this to mean that the magma was main- 

 tained for a considerable time at the temperature requisite for the 

 separation of some particular mineral, and only slightly below that 

 at which some other mineral, present in a very much smaller quantity^ 

 had solidified. Lor instance, in the case of magnetite and leucite, 

 the latter mineral begins to form in a magma in which scattered 

 granules of the former have appeared. Pirst, probably, there is a gene- 

 ral development of microlithic leucites ; next, owing to a slight non- 

 uniformity of conditions, certain of these act as centres of attraction. 

 The first tendency will be for the leucite microliths to aggregate 

 and, in so doing, to exclude the magnetite, if it be only sparingly 

 present ; but after a while the nucleus becomes larger, the magma 

 possibly slightly more viscous. Motion is not quite so free, and the 

 converging microliths of leucite bring with them granules of magne- 

 tite, and, it may be, the enlargement of the nucleus (as mentioned 

 above) facilitates crystal-formation ; hence the granular magnetite is 

 included in the crystal. (It must be remembered that when one 



