218 
THE GF.(JUiGIST. 
fossils only ; the chalk itself is of a creamy colour, and the drawings on it look as 
if done with Indian ink by a very fine pencil. I have observed the same descrip- 
tion of markings on the chalk and fossils of the White Rocks, near Dunluce 
Castle, County Antrim. 
" I send you a small quantity of the dendritic grains, and a little bit of chalk. 
The dra\ving upon it is not so distinct as I could wish ; in some chalk it is per- 
fectly black, and very beautiful indeed, and in all such instances I observe that the 
chalk is of peculiar hardness, while at the same time the markings penetrate 
deeper, with increased depth of colom- as they proceed ; but in the softer chalk the 
tintings are brownish, and more on the surface. 
" Are those moss-hke figiu'es common to all chalk ? 
" 2. Why do those portions of Fentacrinites, commonly called ' star-stones,' 
move with a sensible rotatoiy motion in a vessel of vinegar ; or why do they move 
at all ( And why wiU not a portion of Belemnite, or plain limestone, both from 
the same rock, and of the same size as the ' star-stone,' move also ? — Very truly 
yours, A. DE S. M., Port Stewart." 
1st. The beautiful feathery dendritic markings in chalk and on chalk fossils are 
due to a crystallization of manganese either on the planes of cracks aud fissiu^es, 
in the fine interstices between tlie rock-matrix and the fossils, or filling up minute 
cracks in the structiu-e of the rock itself. Such dendritic markings are found in most 
solid rocks, and have been very commonly attributed to iron. We believe, however, 
that when iron puts on the dendritic fonn it is due to the admixture "vvith it of 
manganese. Where the proportion of iron is larger, the dendritic markings are 
dingy and obscure, and by a few days' exposure to the air diminish to a brown 
stain. Where the manganese predominates, the dendritic ornamentation is shaq), 
clear, and defined, and of a dense black colour. 
We have analysed the sample forwarded, and find that it contains the larger 
proportion of manganese with traces of iron. 
The quantity sent was quite sufficient for a qualitative, but not for a quanti- 
tative, analysis ; so that we caimot state the exact proportion of the two ingie- 
dients. These dendritic markings therefore are due to manganese. 
Very pretty dendritic sculpturings are to be made artificially, by mixing clay with 
a solution of sulphate of copper, and baking or otherwise quickly drying the mass. 
2d. The "Star-stones" move on account of the evolution (by the action of 
the vinegar upon the carbonate of lime of which the pentacrinite plates are com- 
posed) of small globules of carbonic acid gas. Sometimes these may be seen 
clustered round the " star-stone," but often they are so minute, firom the very slow 
action of the vinegar, as to be scarcely visible with a pocket-lens. 
Why bits of limestone or belemnites do not likewise move, may be thus 
explained : — 
The star-stones are of veiy hard and compact stracture, and the gas therefore 
is very slowly evolved by the weak action of the vinegar. From this cause the 
mhmte bubbles congregate under the star-stone, where they are, by the hollows 
and sculjitiuings of which they are retained, and the star-stone, rendered slightly 
buoyant. Then as the bubbles bm-st into each other, and become confluent, or as 
they tilt up fii'st one edge then another of the star-stone in escaping, a slight 
mechanical action is set up sufficient to cause the rotatory motion alluded to by 
om- correspondent. In the case of common belemnites, limestone, &c., the carbonate 
of lime being softer, the gas is generated more quickly and in larger bubbles, 
and consequently readily escapes without at all ser\'ing to float, by clinging to 
it, the object from which it emanates. 
" Dear Sir, — I am frequently applied to by stranger geologists, respecting 
information on the Ledbury and Malvem districts. You would, I think, be 
rendering ramblers a service by informing them that they will find Henry Brooks, 
.shoemaker, the Homend, Ledbury, a most intelligent and efficient giude. He 
has lately discovered there * A uchenaspis SaUerrii,' one of the rarest of the Tilestone 
fossils. — Yours very tnily, W. S. Symonds, Pendock Rectoiy, Ledbury." 
The great Ichthyosaurus platyodon(?) at the York Museum. — "Sir, — 
I was glad to see your correspondent, Mr. S. R. Pattison, bringing forward the subject 
