Plane of 
Milky Way 
620 
there is an increase as we approach the plane of the 
Milky Way. They are, therefore, not limited to the 
plane. Now we know that these stars are the moribund 
stars, the stars just disappearing, the stars whose light is 
waning ; so that soon after the carbon stage they exist in 
the heavens as dark stars, and we can only know their 
existence by their gravitational effect upon other stars 
which are self-luminous. It is also to be borne in mind 
that these stars, just because they are in their waning 
POLE 40 
20 
4O 60 
yo 
Nun bers 
Fic. 2.—Comparison of relative numbers of stars generally and 
carbon stars. 
stage, are very faint ; so that the information we are able 
to get with regard to them may possibly be information 
concerning their distribution in parts of space not very 
far distant from that which we ourselves occupy. 
That was in 1884. In 1891 Prof. Pickering, when he 
found that he had collected something like 10,000 stars 
in the Draper catalogue, began to consider their dis- 
tribution in different parts of space in relation to the 
then classification, which was practically a classification 
founded on hieroglyphics, since we knew very little about 
the chemistry of the different bodies at that time. 
He found that the Milky Way was due to an aggrega- 
tion of white stars, by which he meant, as we now know, 
very hot stars, and the hottest of them, that is the 
gaseous ones, exist more obviously in the Milky Way 
than do the others. The proportional number of proto- 
metallic stars in the Milky Way was greater for the 
fainter stars than for the brighter ones of this kind, and 
that at once suggests a possibility that in the Milky Way 
itself there is a something which absorbs light ; so that the 
apparently brightest stars are not actually the brightest, 
but are more luminous because they have not suffered 
this absorption, and that those which have suffered this 
absorption may be very much further away from us than 
the others of a similar chemistry. He also arrived at this 
extremely important conclusion, namely, that the metallic 
stars, that is, stars like our sun, stars more or less in their 
old age, had no preference for the Milky Way at all, but 
are equally distributed all over the sky. With regard to 
the group of stars known by metallic flutings in their 
spectra, he has no information to give us any more than 
Dunér had, for the reason that their number is small and 
they have not yet been completely studied. 
Only last year this inquiry was carried a stage further by 
Mr. McClean, who not only photographed a considerable 
number of stellar spectra in the northern hemisphere, but 
subsequently went to the Cape of Good Hope in order to 
complete the story with reference to the stars down to 
the third or fourth magnitude, which he could observe 
there. He was very careful to discuss, in relation to the 
Milky Way and certain galactic zones, the distribution of 
the various kinds of stars which he was fortunate enough 
to photograph. 
We notice that if we deal with the gaseous stars the 
numbers in the north and south polar region are small, 
and that the numbers nearer the Milky Way are greater, 
so that finally we can see exactly how these bodies are 
NO. 1565, VOL. 60] 
NATURE 
[OcToOBER 26, 1899 
distributed. If we take the gaseous, that is to say the 
hottest, stars, we find the smallest number in the polar 
regions ; but if we take the metallic stars we find practically 
the largest number, at all events a considerable number, 
in the polar regions. The general result, therefore, is that 
the gaseous stars are mostly confined to the galactic 
zones, the proto-metallic stars are not so confined, that 
is to say, down to about 34 magnitude. What is also 
shown there is that the metallic-fluting stars are prac- 
tically equally distributed over the polar regions and 
over the plane of the Milky Way itself; so that, in that 
respect, we get for these stars very much the equivalent 
of the result arrived at by Dunér, that is to say, they 
have little preference for the Milky Way. 
(To be continued.) 
THE PARENT-ROCK OF THE SOUTH 
AFRICAN DIAMOND. 
IAMONDS were discovered in gravels of the Orange 
River in 1867, and were traced three years later to 
a peculiar earthy material called from its colour “ yellow 
ground” by the miners. This, which was soon found to 
pass down into a more solid and dark-coloured material 
called “blue ground,” occupies ‘‘ pipes” in the country 
rock—carbonaceous shales and grits belonging to the 
Karoo system ; the one standing in much the same relation 
to the other as do the volcanic necks to the carboniferous 
strata in Fifeshire. Flows or sills of basaltic rocks are 
associated with the sedimentary strata, and both are cut 
by dykes. The matrix of the blue ground is a fine 
granular mixture, chiefly consisting of a carbonate 
(calcite or dolomite) and serpentine. In this are em- 
bedded grains of garnet (mostly pyrope), pyroxenes (a 
chrome diopside, smaragdite or enstatite), a brown mica, 
magnetite and other ores of iron, and some other 
minerals more sparsely distributed. Rock fragments 
also occur; some of them are the ordinary shale and 
grit, but others are compact and of an uncertain aspect. 
Crystalline rocks are sometimes found. 
As to the nature of this blue ground and the origin of 
the diamond, very diverse opinions have been expressed. 
The late Prof. Carvill Lewis considered the former to be 
a porphynitic peridotite, more or less serpentinised, which 
sometimes passed into a breccia or a tuff, and the diamond 
to have been formed zm sz¢z by the action of this very 
basic igneous rock upon the carbon present in the Karoo 
beds. Others, however, maintained that the rock was 
truly clastic ; being produced by the explosive destruction 
of the sedimentary rocks, together with part of their 
crystalline floor—was, in fact, a kind of volcanic breccia, 
subsequently altered by the action of percolating water at 
a high temperature. But they also differed in opinion as 
to the genesis of the diamond itself ; one party holding it 
to have been formed 77 szfu, by the action of water at 
a high temperature and pressure, the other considering 
it, like the garnets, pyroxenes, &c., to have been formed 
in some deep-seated holocrystalline rock mass, and to 
have been set free, like them, by explosive action. 
A few months ago the investigation had advanced thus 
far: (1) study of the diamonds obtained from the blue 
ground had increased the probability of their being 
derivative minerals ; (2) no certain proof of the former 
existence of a compact or glassy peridotite had been dis- 
covered ; (3) certain compact rock fragments, as to the 
origin of which the writer had at first hesitated to express 
an opinion, had been determined by him to be only argil- 
lites, affected first by the action of heat, then of water ; 
(4) the diamond and the garnet had been brought into 
very close relation by the discovery of two specimens, 
showing the former apparently embedded in the latter. 
The better of them was accidentally picked up at a depth 
of about 300 feet in a shaft at the Newlands Mines, West 
1 The substance of a paper read before the Royal Society on June 1. 
