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of rock-species, to fall into disuse. For half a century we 
have been content to make shift with vague, incorrect 
names invented in the infancy of the science; our pro- 
gress in this respect since the early days of M‘Culloch, 
Boué, and Jamieson, having been simply wzz/. More 
especially is this true of our nomenclature of igneous 
rocks. While we have unravelled the complicated 
stratigraphical structure and relations of these rocks 
with unrivalled labour and detail, we have left aside the 
questions touching their mineralogical ingredients and 
.chemical composition, and their classification as mineral 
compounds. English petrography does not exist ; what 
we have in its stead is an indefinite obsolete grouping of 
rocks patched up with occasional borrowings from the 
Continent. And yet, strange to say, it is in England that 
the most important step in modern petrography has 
originated. Sorby’s application of the microscope to the 
study of rocks has opened a new era in the science, and 
our good friend Sorby himself is regarded as a kind of 
demi-god in the eyes of our German brethren of the 
hammer. But even his wand, though it has raised up a 
new army of zealous petrographers on the Continent, has, as 
yet, failed to quicken the dry bones of English petrography 
Mr. David Forbes is our sfes altera Rome. Our waiting 
eyes have been turned to his laboratory in York Place 
Portman Square, for years past. His materials are vast, 
his enthusiasm great, and his intention fixed, to retrieve 
the honour of English petrography. May his shadow 
never be less until long after his wishes and our hopes are 
fully realised ! 
They manage things petrographical very differently in 
Germany. There the study of rocks is introduced into the 
curriculum of schools and universities, It is treated of in 
many excellent text-books. It is eagerly pursued by zealous 
investigators from Berlin to Vienna. The great paper of 
Mr. Sorby, published here thirteen years ago, has done 
much to quicken this research by showing that the older 
methods were in many respects untrustworthy. These 
methods were based primarily upon chemical analysis. 
But such analysis, while it reveals the ultimate chemical 
constitution of the rock, may not explain its mineralogical 
composition, The various stages of the metamorphism 
of the component minerals are thereby often lost sight of. 
Hence two rocks, having by analysis approximately the 
same chemical composition, may differ materially from 
each other in mineralogical composition. It is here that, 
as Sorby showed, the microscope comes in to our aid, and 
shows what the different mineral ingredients of the rock 
are, how far they have respectively undergone alteration, 
how they are built into each other so as to form the rock- 
mass, and under what conditions they may originally have 
been formed. This important addition to the methods of 
research has so powerfully affected petrography, that this 
branch of science must be regarded as at present in a 
transition state. Many of the groups of rocks in the 
nomenclature now in vogue in Germany will require re- 
consideration. More especially is revisal needed in those 
based upon subdivisions of the triclinic felspars. Petro- 
graphers are now coming to see that, in a vast number of 
cases, it is not possible to discriminate the particular 
species of felspar in a rock, further than as belonging to 
the othoclase or plagioclase division. In this separation 
the microscope becomes of essential importance, 
NATURE 
[Feb. 9, 1871 
A small pile of German petrographical literature has 
accumulated on our table, and we propose in this and a 
subsequent paper to notice the more important works. 
The first volume that comes to hand is another publication 
of that most voluminous writer, Dr. Ferdinand Senft, 
Professor of Natural, Science, Eisenach. He seems to 
issue a goodly octavo every year, though possibly the past 
year’s political events may have interrupted his labours 
for 1870. The present work is entitled “A Text-book of 
Mineralogy and Petrography,” and contains some 700 
pages. One would have thought that the Doctor had 
hardly left himself room for such a book as this, when 
we remember not only his former special treatises on the 
subject, but his text-book of “ Forstlicher Naturkunde,” 
one of the volumes of which is devoted to geognosy, soils, 
and chemistry. And yet the book differs materially from 
any of his former works, and, if we mistake not, is likely to 
beat least quite as useful. It is not designed to be an elabo- 
rate methodical text-book, but one in which the teacher 
and pupil will find all the material they require for a suc- 
cessful and methodical study of minerals and rocks, and 
also one which will prove sufficient for the student in his 
early inquiries, even without the help of a master. The 
author has had peculiar advantages for the compilation 
of such a book. In an interesting preface he tells us that 
for a quarter of a century he has been engaged in teaching 
these subjects to the two higher classes in a school, and 
he details the method of instruction which his experience 
has found to be successful. He had used the best mine- 
ralogical treatises as text-books in his classes, but had 
always found them too difficult for use in schools, Ac- 
cordingly in 1860 he brought out a little “School Text- 
book of Mineralogy and Geognosy,” which, having been 
out of print since 1866, he has remodelled and enlarged 
into the present work. 
The general plan of the book is like that of the ordinary 
German text-books, only the first or mineralogical division 
occupies about five times more space than that devoted 
to petrography. It is of the latter that we have at present 
to speak. Retaining the usual grouping of crystalline 
and fragmental rocks, the writer gives a clear and succinct 
account of each subdivision and species. His plan for 
the specific details somewhat resembles that in his earlier 
work on the Classification of Rocks, but with some improve- 
ments. Under each species of rock, a clear but brief de- 
scription of its leading features is given in large type, then 
follows an equally concise account of its varieties, tran- 
sitions, mode of weathering, geological occurrence, and 
geographical distribution. The notes on the weathering 
of the different rocks, and the general remarks on that 
subject in the introductory part of the petrographical sec- 
tion, go some way to supply a want which every beginner 
soon discovers to exist in other manuals. The same com- 
mendation may be given to the descriptions of the various 
kinds of débrzs and soil formed by the decay of rocks, 
As a school-book, the present volume seems likely to 
prove useful. The arrangement into short subdivisions, 
each clearly marked in the mode of printing, and treating 
each rock in the same method, will facilitate the progress 
of aclass, and give precision to the inquiries of a beginner. 
Were there only any general taste for such pursuits in our 
own country, we might hope to see the book translated 
and adapted for use in our colleges and schools. At the 
