Grenville Cole—Artificial Perlitic Structure. 115 
minute shells with a “sort of spiral organization by no means unlike 
that of the Planorbis or Spirorbis.”! The figure which accompanies 
this short description renders it quite clear that the body found by 
Hibbert, called by him a Nautilus, could be no other than those to 
which Murchison applied the name Micro. carbonarius. 
Dr. Hibbert’s figure of the Nautilus was reproduced shortly after 
its publication by Mr. W. Rhind in a little work entitled, ‘“ Execur- 
sions illustrative of the Geology and Natural History of the Environs 
of Edinburgh,”? without comment or further information. In the 
Ist edition of his “ Catalogue,” Prof. J. Morris united the Helicites 
pusillus, Martin, and Murchison’s Micro. carbonarius, and placed 
the species in the Gasteropoda. 
In 1848 Col. Portlock recorded* the occurrence of two forms of 
Spirorbis in the Irish Carboniferous rocks, S. omphalodes, Goldf., and 
S. minutus, Portlock. Of the first it is said, ‘they are fixed on the 
compressed stems of Calamites, and if removed, are found to have 
made a. well-marked impression on them.” ‘The second species is 
said to resemble the first in general form, “but are so minute and 
crowded together as to appear like a fine granulation on the crust of 
Dithyrocaris Colei.”” IJ shall have occasion to make some remarks on 
these forms further on under the subdivision ‘ Observations.” 
Following Portlock, we have, in 1844, Prof. M-Coy,t who quotes 
both the preceding species, but places a note of interrogation after 
the determination of S. omphalodes, Goldf. He says, ‘On certain 
shale plants are found abundance of a little shell referred by Captain 
Portlock to the above species. The Irish specimens are perfectly 
flat on the attached side, smooth, and having two and a half or three 
turns in the spire; the mouth semicircular.” 
(To be continued in our next Number.) 
V.—On tHe ArtiriciaL Propuction or THE PERLITIC STRUCTURE. 
By GRENVILLE Coxe, 
Demonstrator in the Geological Laboratory of the Royal School of Mines. 
ARIOUS investigations of the Perlitic structure in igneous 
rocks have resulted in its being regarded as a product of 
unequal contraction in a cooling mass of lava. Professor Bonney, 
in a paper on Columnar, Fissile, and Spheroidal Structure,’ has 
compared it to the roughly-concentric joints to which, in certain 
basalts, the spheroidal character is due; and Mr. 8. Allport, in his 
account of certain Ancient Devitrified Pitchstones and Perlites,® 
writes that “an examination of all the facts leads to the conclusion 
that the perlitic texture is purely a phenomenon of contraction.” 
Of this, then, there has practically been no doubt; but, so far 
as I am aware, the structure has not until recently been imitated 
by artificial means. 
1 Trans. R. Soc. Edinb, 1866, xiii. p. 151. 2 12mo. 1836, p. 35. 
3 Geol. Rept. Londonderry, etc., 1843, p. 363. 
4 Synop. Carb. Lime. Foss. Ireland, 1444, p. 170. * 
5 Quarterly Journal of the Geological Society, vol. xxxii. p. 149, 
6 Ibid, vol. xxxiii. p. 461. 
