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[March 15, 1883 
they are found to answer the requirements necessary for 
exactitude, 
For these purposes the following apparatus has been devised 
by the author, and brought into use at the Observatory. 
A telescope of 3} inches aperture and 48 inches focal length, 
a pair of collimators of 13 inch aperture and 10 inches focal 
length, and a heliostat, are firmly fixed to a stout plank, so that 
their axes may be in the same horizontal plane. The eyepiece 
of the telescope carries a parallel wire micrometer. 
In order to adjust the instrument, the telescope is directed to 
the sun, a shade being fitted to the eyepiece and then placed in 
its Y’s focused for parallel rays. The collimators are then 
fixed on their table with their object-glasses opposed to that of 
the telescope, the eyepieces and wires having first been removed, 
and a metal plate with a sharply-cut hole in its centre fitted to 
their diaphragms. 
Light is next reflected down the collimator by the heliostat, 
and the aperture in the diaphragm being viewed through the 
telescope, is carefully focused by moving the object-glass of the 
collimator to and fro by means of its rack and pinion. 
The diaphragm aperture is next collimated by rotating the 
collimator in its bearings. 
Both collimators being thus adjusted, they are placed sice by 
side, so that their illuminated sights can be viewed simulta- 
neously in the telescope, appearing as superimposed bright disks 
12’ in diameter. They are next separated so that the disks 
remain merely in contact at the extremity of their horizontal 
diameters. ; 
The instrument is now ready for use, and the examination of 
the shades is performed in the following manner :— 
The glass to be tested is fixed in a rotating frame in front of 
the object-glass of one collimator, a corresponding shade being 
placed between the heliostat and diaphragm of the other colli- 
mator. The sun is now directed on to the diaphragms. The 
coloured disks are viewed through the telescope, when, if the 
sides of the shade, placed between the collimator and the object- 
glass of the telescope, are perfectly parallel, the relative posi- 
tion of the disks is unchanged ; if, however, the shade is not 
ground true, the disks will a; pear either separated or to overla». 
In the first case the amount of separation is measured by the 
micrometer, and serves to indicate the quality of the glass. In 
the case of overlapping images the shade is rotated through 180°, 
and separation produced which can be measured. A second 
examination is then made, the shade having been turned through 
go°. 
If in no position a separation of images is found to exist to 
the extent of 20”, the glass is etched K.O. 1; if more than 
20" but less than 4o", the mark is K.O. 2; with greater distortion 
than this, the shade is rejected and not marked. 
To examine the quality of the mirrors, a small table, on 
levelling screws, is put in front of the object-glass of the tele- 
scope. The mirror to be tested is placed on its edge on this 
table, and turned until a distant well-defined object is reflected 
down the tube of the telescope. The object-glass of the tele- 
scope having previously been stopped down to an aperture 
corresponding to the size of the mirror, the reflected image is 
contrasted with that seen directly, and if the definition is un- 
changed the mirror is marked K.O, with a writing diamond, and 
returned to the maker ; if the object appears distorted, its un- 
fitness for use is similarly notified. A small fee is charged for the 
examination. 
Geological Society, February 7.—J. W. Hulke, F.R.S., 
president, in the chair.—G. D’Arcy Adams, Prof. Ferdinand 
Moritz Krausé, and the Rev. Alfred William Rowe were elected 
Fellows, and Dr. Karl A. Zittel, of Munich, a Foreign Corre- 
spondent of the Society.—The following communications were 
read :—On the metamorphic and overlying rocks in parts of 
Ross and Inverness shires, by Henry Hicks, F.G.S., with 
petrological notes by Prof. T. G. Bonney, F.R.S. In this 
paper the author described numerous sections which have been 
examined by him in three separate visits made to the north-west 
Highlands. In some previous papers, sections in the neighbour- 
hood of Loch Maree had been chiefly referred to. Those now 
described are to the south and south-east of that area, and occur 
in the neighbourhoods of Achmashellach, Strathcarron, Loch 
Carron, Loch Kishorn, Attadale, Strome Ferry, Loch Alsh, 
and in the more central areas about Loch Shiel and Loch Eil to 
the Caledonian Canale In these examinations the author paid 
special attention to the stratigraphical evidence, to see whether 
there were any indications which could in any way be relied” 
upon to prove the theory propounded by Sir k. Murchison that 
in these areas fossiliferous Lower Silurian rocks dip under thou- 
sands of feet of the highly crystalline schists which form the 
mountains in the more central areas. On careful examination 
he found that in consequence of frequent dislocations in the 
strata the newer rocks were frequently made to appear to dip 
under the highly crystalline series to the east, though in reality 
the appearance in each case was easily seen to be due to acci- 
dental causes. Evidences of dislocation along this line were 
most marked; and the same rocks in consequence’ were 
seldom found brought together. He recognised in these eastern 
areas at least two great groups of crystalline schists metamor- 
phosed throughout in all the districts examined, even when 
regularly bedded and not disturbed or contorted ; and they have 
representatives in the we-tern areas, among the Hebridean series, 
which cannot inany way be differentiated from them. These he 
called locally by the names, in descending order, of Ben-Fyn, 
and Loch-Shiel series. ‘lhe former consist, in their upper part, 
of silvery mica-schists and gneisses, with white felspar and 
quartz ; in their lower part, of hornblendic rocks, with bands of 
pink felspar and quartz, and of chloritic and epidctic rocks and 
schists. The Loch-Shiel series consists chiefly of massive grani- 
toid gneisses and hornblendic and black mica-schists. Thirty- 
three microscopical sections of the crystalline schists and the over- 
lying rocks are described by Prof. Bonney, and he recognises 
amongst them three well-marked types. In No. 1 he includes 
the Torridon Sandstone, the quartzites and the supposed over- 
lying flaggy beds on the east side of Glen Laggan, “These are 
partially metamorphosed,’ only distinct fragments are always 
easily recognisable in them in abundance, In No. 2, the Ben- 
Fyn type, the rocks are crystalline throughout, being typical 
gneisses and mica-schists. In No. 3, the Loch-Shiel series, he 
recognises highly typical granitic gneisses of the Lower Hebri- 
dean type. Dr. Hicks failed to find in these areas at any point 
the actual passage from group I to group 2; neither did 
the same rocks belonging to group I meet usually the same 
rocks belonging to group 2. The evidence everywhere showed 
clearly that the contacts between these two groups were either 
prodtced by faults or by overlapping. Group 3, placed 
by Murchison as the highest beds in a synclinal trough, 
supported by the fossiliferous rocks, the author regarded 
as composed of the oldest rocks in a broken anticlinal. 
They are the most highly crystalline rocks in these areas ; 
and the beds of group 2 are thrown off on either side in broken 
folds. These, again, support the rocks belonging to group I. 
The author therefore feels perfectly satisfied that the crystalline 
schists belonging to groups 2 and 3, which compose the moun- 
tains in the central areas, do not repose conformably upon the 
Lower Silurian rocks of the north-west areas with fossils, and 
that these highly-crystalline rocks cannot therefore be the meta- 
morphosed equivalents of the comparatively unaltered, yet highly 
disturbed and crumpled, richly fossiliferous Silurian strata of 
the southern Highlands, but are, like other truly crystalline 
schists examined by him in the British Isles, evidently of pre- 
Cambrian age. In an Appendix by Prof. T. G. Bonney, 
F.R.S., on the lithological characters of a series of Scotch 
rocks collected by Dr. Hicks, the author stated that he observed 
in the above series, as he had done in other Scotch rocks lately 
examined by him, three rather well-marked types—one, where, 
though there is a certain amount of metamorphism among the 
finer constituents forming the matrix, all the larger greins, 
quartz, felspar, and perhaps mica, are of clastic origin; a 
second, while preserving a bedded structure and never likely to 
be mistaken for an igneous rock, being indubitably of clastic 
origin, retains no certain trace of original fragments ; while the 
third, the typical ‘‘old gneiss” of the Hebridean region, 
seldom exhibits well-marked foliation. It is sometimes difficult 
to distinguish between the first and second of these; but this 
the author believed to be generally due to the extraordinary 
amount of pressure which some of these Scotch rocks have 
undergone, which makes it very hard to determine precisely 
what structures are original. Even the coarse gneiss is some- 
times locally crushed into a schistose rock of comparatively 
modern aspect. ‘The least altered of the above series the author 
considered to be the true ‘‘newer-gneiss” series of the High- 
lands, but both of the others to be much older than the Torridon 
Sandstone.—On the Lower Carboniferous rocks in the Forest of 
Dean, as represented in typical sections at Drybrook, by E. 
Wethered, F.G.S., with an appendix by Dr. Thomas Wright. 
