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undulated and bent streams, damming up before a larger crystal, and flowing around it 
to unite on tbe other side (giving the crystal something the appearance of an eye)> 
often also really scattered and dissipated by one of them. These appearances 
evidently indicate that the fluctuations happened in the stiffening glass magma, after 
the microlites or little needle-formed crystals had been solidified. Analogous pheno- 
mena of motion, fluctuation or fluidal structure, invisible to the naked eye in the hand 
specimens, are very often observed in the thin sections of partly or almost wholly 
crystalline massive rocks, such as basalts, trachytes, phouolites, melaphyres, and green- 
stones. The smallest ledge-formed sections of orthoclastie or plagioclastic felspars, 
prisms of hornblende or augite, microlites of a variety of kinds ; in short, all the 
microscopical bodies possessing a longitudinal axis, are locally grouped parallel to one 
another, and form undulating streams which diverge in the form of fans or ice-flowers. 
Where larger crystals lie in the paths of these crowded bands, the little needle-formed 
crystals encircle them on all sides with a tangential arrangement, are turned aside into 
different paths, or come to an abrupt end before them, as if by a shock, the microlites 
being thrown asunder in all directions. Observations of these phenomena of fluidal 
microstructurc are best made between crossed nicols, for the single crystals are then 
coloured, and exhibit their characteristic direction much better than in ordinary light. 
A low magnifying power best enables one to overlook at once a larger portion of the 
then section, and thereby to follow the lines of fluctuation. The shape of the little 
crystals is not without importance in the distinct obsci’vation of the form of the fluctua- 
tions. If they are needle-like or ledge-formed, even feeble movements of the mass will 
be unmistakably expressed ; if, on the contrary, they are of a roundish, granular form, 
it often happens that strong fluctuations which have taken place fail to leave a trace of 
their action. In some rocks, especially the rhyolites, this wavy structure is produced 
by small dark grains grouped into lines and bands. These lines of grains undulate in a 
most remarkable mannei', so that the figures of their curvature resemble marbled paper. 
There are also curled and twisted stripes of felsitic material, differing in colour and 
behaviour, which render the wa^nng motion evident. 
“ Three important points present themselves upon which light is thrown by this 
remarkable microstructure, connected with the fluctuations of the solidifying mass. It 
proves that the rock was at one time a magtna, in a plastic state, and that, after larger 
crystals had been secreted, a shifting and displacement of the small microlites happened. 
Soon afterward the mass seems to have been so suddenly solidified that the streams 
became fixed, and their fluctuation preserved for our observation. And, from these 
facts, the conclusion follows that the large and small crystals -were not formed e.\actly 
where we perceive them, but that they have been thrown into their present place by the 
purely mechanical action of the surrounding plastic mass. It is worth mentioning that 
those rocks whose microfluidal structure is particularly distinct, are generally propor- 
tionately rich in broken crystals, shivered into detatehed, sharply angular fragments. 
And, lasily, this structure proves that the smallest crystals of the rock have not altered 
their mutual grouping and form, which date back to their solidification ; and that, 
although secondary decompositions may have ocem-red in the lapse of time, these 
metamorphic influences have by no means been sufficient to obliterate the original 
characteristic structure.” 
The word “rhyolite” is compounded from /5ua| (a lava stream) , and Xidos (a stone), 
and was first introduced by Richthofen in 1860. 
I was somewhat exercised as to the inclusion of the quartz-trachytes of Mackay 
in the group, but Eutley says * : — “ Some petrologists include obsidian, pitchstone. 
* “ The Study of Rooks,” p. 177. 
