538 
SILURIA. 
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 i 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 similar to the so-called 
' 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 
balls of hornstone occur, quite similar to the so-called felspathic lava of Snowdon, 
which perhaps is only a more quartzose portion of the green slate. The ' felspathic 
trap ' of Snowdon is quartzose schist : the hollows in it, partially filled with 
iron-ore, are derived from decomposed included minerals, the nature of which 
is no longer to be ascertained. The 1 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 with 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. CaeeutheeSj F.L.S. 
[The great importance of the Graptolite as a peculiarly Silurian 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 following 
Note on Graptolites by Mr. Carruthers (one of the few persons who have espe- 
cially studied them) of peculiar interest to the readers of ' Siluria.'] 
The genus Graptolithus was established by Linnaeus in the first edition of his 
1 Systema Naturae ' 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 until the 
twelfth edition ; and in this we find a double-celled species (G. scalaris), which 
Linnaeus had already figured in his ' Scanian Travels.' This is the true type of 
the family, and the only species with which Linnaeus was acquainted. The 
single-celled Graptolite which has by every one been referred to Linnaeus's 
G. Sagittarius, 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 error, and to make the extent of the acquaintance which Lin- 
naeus had with these fossils 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 Graptolites are generally found, and their 
somewhat anomalous structure, have caused many different opinions to be enter- 
tained regarding their 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 — 
