^^^                                                  SILUEIA.

partly changed into uralite. The ' felspar-porphyry,' from Caernarvonshire, is
a syenite-porphyry with imbedded crystals of felspar and mica: oligoclase and
hornblende also occur occasionally in addition. This rock is connected, as to
its character, with the ' syenite' of Caernarvonshire : the ' syenite' of Merion¬
ethshire, on the other hand, contains much quartz, and has a more granitic
character. The ^ hornblende-porphyry' is a diorite very simUar to the so-caUed
' Tigre-ore' of Schemnitz ; but where the imbedded balls of hornblende are much
larger, it is more like the diorite of Raschewsk in the Ural.

^' The ' ash' of Snowdon and of Bala and the ' felspathic lava' of Snowdon are
varieties of the green slate: in that of Snowdon are concretions resembling
those of the so-called ^ fruit-slate' (Frucht-Schiefer) of Saxony; in that of Bala
baUs of hornstone occur, quite similar to the so-called felspathic lava of Snowdon,
which perhaps is only a more quartzose portion ofthe green slate. The ' felspathic
trap' of Snowdon is quartzose schist: the hollows in it, partially fiUed with
iron-ore, are derived from decomposed included minerals, the nature of which
is no longer to be ascertained. The ' felspathic ash' of Bala contains in its mass
(or paste), which may be scratched with a knife, very small white crystals of
felspar: it appears to be only a variety of the ordinary porphyry; but we ought
to be able to see more of the rock, and to be acquainted vsdth its geological
conditions, in order to form a decided opinion on it. The ' felspathic ash' of
Bishop's Castle is perhaps something similar, only more decomposed."

D.—On Graptolites.    By W. Caeeuthers, E.L.S.

[The great importance of the GraptoHte as a peculiarly SUurian fossil (see
pages 61 &c.), and the great advance of late years in our palaeontological know¬
ledge of the various forms of this interesting organism, will render the foUov^dng
Note on Graptolites by Mr. Carruthers (one of the few persons who have espe¬
cially studied them) of pecuHar interest to the readers of ^Silm-ia.']

The genus Graptolithus was estabhshed by Linnaeus in the first edition of his
' Systema Natm-ae ' for a series of natural productions which had previously been
considered to be true fossils. In the genus as it appeared in the early editions
of that work not a single species of the fossils to which the name is now con¬
fined was included. No alteration was introduced into the genus untU the
twelfth edition; and in this we find a double-ceUed species (G. scalaris), which
Linnaeus had already figured in his ' Scanian Travels.' This is the true type of
the famUy, and the only species with which Linnaeus was acquainted. The
single-ceUed GraptoHte which has by every one been referred to Linn^us's
G.^ sagittaiius, has nothing whatever to do with the organism to which he gave
this name. His species is founded on a fragment of Lepidodendron figured by
Volkmann; and a perusal of the characters given by Linnaeus makes it evident
that this figure was all the material he had for the establishment of his species.
To correct this en-or, and to make the extent of the acquaintance which Lin¬
naeus had with these fossUs more obvious, I propose to substitute for G. Sagit¬
tarius the name of G. Hisingeri, after the distinguished palaeontologist who first
described the species, but erroneously ascribed to it the Linnean name.

The compressed condition in which Graptohtes are generally foimd, and their
somewhat anomalous structure, have caused many different opinions to be enter¬
tained regarding theii- nature. Linnaeus considered that they were natural
imitations of fossils. Bromel, and after him Brongniart, and some of the early
American observers thought they were fragments of different kinds of Plants—