Mr. Macgillivray’s Account of the Outer Hebrides. 411 
country can offer such facilities for investigation to any excepting a 
fire-side mineralogist. 
Excepting quartz rock and hypersthene greenstone, I know of 
no rock so indestructable as the ordinary gneiss of these islands. 
No one can contemplate the mountains of Barray and Harris in 
particular, without perceiving that they still present the very sur- 
faces with which they first were exposed to the atmosphere. On 
these mountains there are no accumulation of debris, no alluvial 
substances, no mineral soil. The bare and bleached rock is merely 
covered here and there by shreds of peaty soil, in which a scanty 
vegetation supports itself. Great blocks sometimes present them- 
selves, but they are in all cases traceable to the neighbouring moun- 
tains. Of diluvium I have seen none in any part of the range, al- 
though in some places, especially in Harris and Lewis, deposits of 
clayey matter, mixed with angular, and sometimes rounded frag- 
ments of rocks, present themselves in the glens and along the 
shores. Towards the bases of the mountains there is generally, 
however, a layer of clay of a greenish colour, containing angular 
pieces of stone, and apparently derived from the decomposition of 
the felspar, which forms so abundant an ingredient in the rocks ; 
and in some few glens I have seen a kind of gravel, which how- 
ever had always more angular than rounded fragments in it. Upon 
this clay or gravel, and upon the bare rock, rest the peat and sand 
which form the almost only soils of the Outer Hebrides. Of these 
I intend to speak in the next section. 
(To be continued.) 
ART. If. A Monograph on the Pisipium, a New Genus of 
British Fresh Water Testacea. By Carr. THomas Brown, 
F.R.S.E. F.L.S. &c. &c. 
THE genus Pisidiwm was instituted by Carl Pfeiffer in his 
“ Land-und-Wasser Schnecken,” published at Berlin in 1821. 
His generic type is the Cyclas obliqua of Lamarck, or Tellina am- 
nica of the Linnean arrangement, which was the only species of 
Pisidium known to inhabit Britain, until I noticed the P. fontinale 
in my paper on some new British testacea, in the first No. of this 
Journal, page 11, which I described as a Cyclas, not having then 
had an opportunity of examining the animal. In.the end of last 
autumn I met with the third and only remaining species of Pfeif- 
fer’s genus near this city. I have kept specimens alive in water ever 
since ; this has enabled me to ascertain the form of the animal, 
which is quite different from that of the Cyclas. The following 
are the generic distinctions of Pfeiffer :— | 
