578 
NATURE 
» 
| April i9, 188 
metals, and concludes by dealing with the behaviour and 
action of the larger machine tools, including planing, 
shaping, and slotting machines, all of which are illus- 
trated. 
The following chapters go fully and minutely into the 
construction and working of the lathe, perhaps the most 
important of all the machines in a workshop; describe 
its various uses, both for hand-turning and wood, and the 
mechanical slide-rest for metals. A screw-cutting and 
surfacing lathe is illustrated, and all its different motions 
explained. It is impossible here to do justice to these 
chapters on the lathe and turning in general. The student 
will find the time well employed if he studies them care- 
fully, the author evidently being well acquainted with the 
practical working of this all-important machine. 
The remaining chapters are occupied with drilling, 
boring, and the necessary machines for carrying out the 
same ; a variety of drills are shown, and their different 
uses explained. The drilling machine, its construction, 
and various arrangements of feed gear are illustrated and 
concisely shown. 
The book concludes with shearing and punching 
machines.. Various illustrations are given, including 
Whitworth’s driving gear for the same, and an illustra- 
tion of Tweddle’s hydraulic shearing and punching 
machine. 
As a treatise on cutting-tools, for wood and iron, this 
work will be found extremely useful to engineers gene- 
raliy ; moreover, there is a good deal of original informa- 
tion that will be found interesting to experienced tool- 
makers. The classification of the machinery is decidedly 
good, and the descriptions are so simple as to be easily 
understood by the uninitiated. It is impossible to study 
the book without at once finding that the author is com- 
pletely master of his subject. Of course there is no 
doubt that practical working is essential to perfection in 
any branch of engineering ; yet the student who is unable 
to attain such practical knowledge will obtain a good 
insight into the construction and the uses of the various 
machines and tools in their connection. 
The author certainly seems to have omitted a matter 
of great importance in tool-making, namely, that of tem- | 
pering and hardening the cutting-tools. There is little 
doubt that most of the failures arising in wood and iron- 
working machinery are due to tools not being properly 
hardened. A chapter or two devoted to the subject would 
have been of great service. 
LETTERS TO THE EDITOR 
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No notice ts taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space ts so great 
that it ts impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts.| 
Metamorphic Origin of Granite.—Prehistoric ‘‘ Giants” 
I HAVE for some time intended tosend you a few notes on two 
matters, bo'h connected with geology, though very different in 
kind. In Nature, vol. xxvii. p. 121, there was an interesting 
pap-r by Mr. Geikie on the metamorphic origin of granite 
and on the crystalline schists. Last autumn I became satisfied 
of aconclusion which | had long suspected—that the large granitic 
district in the Ross of Mull, adjacent to the Island of Iona, 
is a great mass of granite formed by the metamorphism of an old 
stratified deposit belonging to crystalline schists. It is well 
known to geologists that the Isle of Mull consists almost entirely 
of the series of (Tertiary) volcanic rocks which have been admir- 
ably described by Prof. Judd. These traps, tuffs, and lavas rest 
in sonie places on chalk, and where the chalk had been previously 
denuded they rest on Ovlitic and Liassic beds. Older rocks, be- 
longing, I think, to the Cambrian series, appear at one place 
subjacent to the traps. But the limit of all these volcanic rocks 
to the south-west is sharply defined by the deep bay and harbo 
of Bunessan, called Loch Laigh. As we enter that loch in | 
boat, we have on our left the trap headland of Ardtun, where 
found the Tertiary leaf-bed many years ago, and on the right a 
headland of massive red granite. But the shores at the end or 
head of the bay, including all the hills above the village of 
Bunessan, are neither trap nor granite, but are composed of thi 
regular crystalline mica schists which constitute the great bulk 
the county of Argyll. ‘This is the only part of Mull, so far as 
know, where these rocks appear. They stretch right across the 
long promontory of the Ross to the southern shore. At the hi 
of Loch Laigh they are more highly crystalline than in most 
parts of the mainland of the county. Very fine crystals o 
tourmaline have been found above Bunessan, and the schists near 
the new pier are highly micaceous and in some places full of 
coarse garnets. These schists dip at a high angle, and indeed 
are in some places nearly perpendicular. On the southern coast 
of the promontory (which is here very narrow) they occupy 2 
considerable space between the traps which terminate on the 
farm of Scoor, and the granite which begins on the farm of 
Ardalanish. The point of contact between these schists and the 
granite is obscured at the head of Loch Laigh, and I have not 
visited it on the southern shore. But the point of most interest 
will be found in the granitic headland which forms the 
south-western shore of Loch Laigh. Along part of this 
shore the granitic masses at the top of the hill have all 
the appearance of standing upon legs. These legs at a 
little distance seem granitic, and although they have a suspi- 
cious appearance of tilted strata, I had passed them over and 
over again under a general impression that they were nothing — 
but granite divided by unusually narrow lines of cleavage. On 
examining them, however, carefully, in August, 1882, I found 
that they are (in my judgment) beyond all doubt crystalline 
bedded schists exhibiting the phenomena of metamorphism in 
the most curious and instructive form. The metamorphicaction 
has often segregated the mineral const'tuents of the old sedi- 
mentary rock in bands tran-verse to the line of bedding, so that 
in one stratum we have bands of pure quartzite and of horn- 
blende gneiss, between bands of granitoid and of pure granitic 
composition. These beds pass up without a break into the 
am rphous granite of the great bulk of the hill; and how pure 
and typical that granite will be acknowledged when I add that 
the columns of the memorial to the Prince Consort in Hyde Park 
are made of it. Since discovering this passage I have found 
some other spots on the c .ast where the relstion of the two rocks 
to each other is well seen. ‘he best is on the deeply indented 
shore of the fam of Awockvoligan, behind the Island of 
**Gilan Giraid,”’ on which the Northern Light Commissioners 
have placed their establishment in the Sound of Iona. Boats 
can be bired at Iona, and at high tide there is a beautiful passage 
behind Gilan Giraid to the shore I refer to. There.a dark 
hornblendic gneiss will be seen underlying, involved in, and 
pas-ing into granite in every form of complication and variety. 
An interesting question arises as to the horizon to which this 
hornblendic rock belongs. As Iona belongs unquestionably, as 
I believe, to the Laurentian series, and the Bunessan schists to 
the metamorpho-ed Silurian, the sub-granite gneiss which 
intervenes may be assigned to either the one or the other. My 
impres-ion is that it represents some of those gneissose beds of 
the Silurian series which are highly developed in Sutherland, 
and lie high above the ‘‘fundamental” or Laurentian gneiss, so 
well known in that county. I should be very glad if some com- 
petent geologist could investigate this district of the Ross of 
Mull, and could confirm or check my observations. 
Turning now to the other subjec>. I have been surprised to 
sée in the English scientific j »urnals no notice taken of the very 
remarkable discovery reported from the Californian Academy 
of Science in a paper communicated to that body by Charles 
Drayton Gibbs, C.E., on the discovery of a great number of 
(apparently) human footprints of a gigantic size in the State of 
Nevada. It appears that in building the State Prison, near 
Carson City, the capital of that State, there was occasion to cut 
into a rock composed of alternate layers of sandstone and clay. 
On several of the clay floors exposed in this operation great 
numbers of tracks of all sorts of animals have been exposed. 
These tracks include footprints of the mammoth or of some animal 
like it, of some smaller quadrupeds apparently canine and feline, 
and of numerous birds. Associated with these are repeated 
tracks of footsteps, which all who have seen are agreed can 
