AND THE DEVELOPMENT OF PERLITIC STRUCTURE, 345 
some larger body. As for the cracks, those have generally been 
formed after the porphyritic crystals, but often before the spherules ; 
otherwise why should the spherules cluster along them? We do not 
always know whether they have been developed exactly where we 
find them, nor do we know what facilities the rock may have 
afforded for their locomotion at different periods of its history. What 
we really do know trom actual observation is this :— 
That.a rock may consist entirely or almost entirely of spherules. 
That a rock may have spherules irregularly distributed through it. 
That the spherules may range themselves in definite lines without 
any apparent guide-line, as we see in some vitreous lavas; in fact 
the spherules floated at certain levels 
That the spherules may collect around isolated crystals or around 
groups of crystals. 
That they may collect along cracks. 
That in some vitreous rocks their development almost exclusively 
along cracks indicates that they have been formed subsequently to 
the solidification of the rock. 
In other instances where they are elongated, as in some lava-flows, 
the evidence is equally strong to prove that they were formed be- 
fore the lava solidified. 
Finally, spherules may: cause the the devitrification of a rock after 
it has solidified and after perlitic fission has supervened. Of this I 
lay before you the most incontestable evidence in a section of de- 
vitrified perlitic obsidian *, now felstone, from the northern end of 
the Long Sleddale valley, in Westmoreland (Pl. XVIII. fig. 6). Itis 
a lava associated with the Coniston Limestone. In the microscopic 
section may be seen :— 
1. Fluxion-bands. 
2. Perlitic structure traversing these bands. : 
3. Minute spherules constituting the whole rock, so far as 
spherules can do so, and passing through the perlitic fissures. 
4, Subsequent fractures. | 
5. Formation of quartz veins along these linés of fracture. 
EXPLANATION OF PLATE XVIII. 
(In each case the nicols are crossed thus, +) 
Figs. 1, 2, 3. Depolarization around crystals in obsidian from Java: x 32. 
Fig. 4. Depolarization, the depolarization ending abruptly against a perlitic 
erack, which only partly surrounds the erystal: x 32. 
4a. The same crystal and crack seen by ordinary transmitted light: x 32. 
5. Orystal completely surrounded by perlitic crack which bounds depola- 
rization-area: X 32. 
* Since perlitic structures are met with in all vitreous rocks, it would 
perhaps be well if the term perlite were abolished as a rock name, and the 
adjective perlitic prefixed instead to the rocks in which such structure occurs. 
This, indeed, is suggested to some extent by Prof. Judd and Mr. Cole in their 
paper upon basalt-glass,; but they there propose to retain the noun instead of 
employing the adjective. 
