Feb. 16, 1871] 
NATURE 
303 


of the rocks, as made known by microscopic analysis ; 
and taking the non-ferruginous, colourless silicate as his 
guide, finds that the rocks hitherto classed under the 
general term basalt, group themselves naturally in three 
divisions: (1) the Felspar-basalts; (2) the Leucite- 
basalts ; and (3) the Nepheline-basalts. All the three 
groups always contain augite and magnetic or titaniferous 
iron, and almost always olivine. So far as Dr. Zirkel’s 
researches go, it appears that all our British basalt-rocks 
belong to the first or felspar group. 
In the last few pages of his memoir the author adds 
some pertinent remarks on the hitherto vaguely defined 
series of rocks, which, as he remarks, under the various 
names of greenstone, trap, melaphyre, &c., play among 
the secondary and palzeozic formations a part like that 
which is performed by the basalts in the tertiary for- 
mations. And here let us remark that in the chrono- 
logical separation of igneous rocks made use of by our 
German fellow-workers, there is something eminently 
unsatisfactory. The term basalt is restricted by them to 
tertiary and post-tertiary rocks. But by what methods 
has the age of eachrock been determined? No geologist 
who has ever had any experience in mapping a district 
of igneous rocks, can fail to realise how exceedingly diffi- 
cult it sometimes is to decide upon the true age of such 
rocks. It is of course easy to say that all basalt is of 
tertiary or post-tertiary date, and, regarding this as an 
axiom, to look on every mass of basalt as of later origin 
than the secondary formations. But the axiom seems to 
us exceedingly doubtful. In Dr. Zirkel’s memoir itself, 
we have basaltic rocks described which are not only 
certainly not tertiary, but are probably paleozoic. That 
igneous rocks have varied in the geological past is highly 
probable, but geologists are hardly yet in a position dog- 
matically to assert that no basalt was ever erupted before 
tertiary times. .We cordially wish that our excellent 
friend Dr. Zirkel will take up the so-called melaphyres ; and 
from what we have ourselves seen of the microscopic 
structure of the British examples, we shall be greatly sur- 
prised if he does not find that from these rocks to true 
basalt there is such an insensible gradation that no sharp 
line can be drawn between them. In the meanwhile he 
deserves the thanks and congratulations of all lovers of 
mineralogical geology for this admirable memoir on the 
basalts. 
That Prof. Zirkel is still busy with his researches, is 
shown by the paper (second in the list at the head of 
this article) which appeared in a recent part of the ewes 
Fahrbuch, and in which he investigates the peculiarities in 
the minute structure of rock-forming minerals, and also 
of artificially-fused basalt and syenite. 
If the limits of this journal and the patience of its 
readers permitted, a good deal ought to be said about 
Roth’s most laborious work on the Petrography of the 
Plutonic Rocks. It is based on the analyses published 
from 1861 to 1868, which, given in full as an appendix, 
form half of the book. The word plutonic is used by 
the author in the sense of originating from igneous fusion, 
and he includes under it, not only igneous rocks commonly 
so called, but also gneiss, schist, and clay-slate. These, 
according to his view, are “the first crusts formed by the 
cooling of the earth’s mass, not metamorphic, that is, not 
altered in various ways by dark, strange processes which 
Sy ee 
appeared but once and never afterwards; although, 
indeed, these gneisses and schists, like other rocks, and 
even more than other rocks, by virtue of their antiquity 
and position, have undergone chemical changes.” From 
this extract one may judge of Herr Roth’s geology. He 
is a chemist rather than a geologist, and has gained 
deserved distinction for the great labour he has ex- 
pended in the collection and discussion of analyses. In 
his present work, read as a memoir before the Berlin 
Academy of Sciences, he has amassed all the analyses he 
could find, which have appeared since the publication in 
1861 of his Gesteznsanalysen, and has prefixed to them a 
discussion of the chemical composition of the various 
rock-species. As a work of reference in the chemical 
part of petrography, the book is of great value. Two 
important features are the analyses of decomposed rocks, 
and the account given of weathering. 
Herr Voglesang is another ardent student of the micro- 
scopic structure of rocks. A few years ago he published 
a little work containing the most beautiful coloured illus- 
trations of that structure which have yet appeared. In the 
present paper he describes under the name of crystallites 
the non-crystallised but yet more or less regularly grouped 
inorganic bodies which are found in crystals and rocks. 
As the paper, however, is to be followed by others, we 
reserve our notice of it for the present. 
Professor Fischer's little pamphlet is a modest produc- 
tion, but one which could not have been prepared without 
a great deal of hard work. Finding that minerals, which 
to all outward appearance are simple and homogeneous, 
can yet be resolved by microscopic examination into as 
many as sometimes four distinct minerals, he has analysed 
by this method some sixty minerals, and publishes his 
results in the present paper, which should be in the hands 
of every petrographer. 
Professor Tschermak’s essay shows how by microsco- 
pical examination with polarised light, it is possible to 
distinguish augite and hornblende, even when minutely 
diffused through a rock. The paper is too important to be 
noticed at the end of this article, and we propose to re- 
turn to it on a future occasion. ARCH. GEIKIE 

GODMAN’S NATURAL HISTORY OF THE 
AZORES 
Natural History of the Azores or Western Tslands, 
By Frederick Ducane Godman, F.L.S., F.Z.S. (Van 
Voorst, 1870.) 
INCE the time when Mr. Darwin called attention to 
the peculiarities of the fauna and flora of the 
Galapagos in his “Journal of Researches,” and showed 
in his “ Origin of Species” how important were the 
lessons to be learnt from oceanic islands in general, the 
subject has had great attractions for naturalists, and 
much material has been collected for its elucidation. Mr. 
Wollaston’s bulky volumes on the Coleoptera of Madeira, 
the Canaries, and the Cape de Verdes, are models of care- 
ful research; but Mr. Godman appears to be the first 
who has, after a personal exploration of one of these 
oceanic groups, endeavoured to collect all that is known 
of its natural productions, and published the result in a 
condensed and convenient form; and for so doing he 
deserves the thanks of all naturalists. 
