24 
POPULAR SCIENCE REVIEW. 
obtained, their probable mode of formation, and the methods 
of cutting and polishing — are the subjects which it will be our 
business to explain in the present article. 
As the tourist ascends the Ehine, and is about to leave one 
of the most picturesque parts of the river for the broad valley 
of the Rheingau, he passes, opposite to the vine-clad hill of the 
Niederwald, the mouth of the River Nahe. This river opens 
into the Rhine on its left bank, just below Bingen, and a little 
above the well-known Mouse Tower. The visitor will find the 
valley of the Nahe almost as beautiful as that of the Moselle, 
to which it runs nearly parallel, the two valleys being separated 
by the Devonian rocks of the Hundsriick Hills. Some distance 
after passing Kreuznach, with its baths, and the neighbouring 
salt-works, the explorer, following the windings of the river, 
reaches the picturesquely seated town of Oberstein, about forty 
miles from Bingen. It is this little town which has been, 
time out of mind, the great centre of the agate trade of the 
world. Although situated in the southern part of Rhenish 
Prussia, Oberstein and the rest of Birkenfeld form an isolated 
patch belonging to the Grand Duchy of Oldenburg — a kind of 
political outlier of the far-distant Duchy, entirely distinct from 
the surrounding Rhine Province. 
Few branches of industry owe their birth more directly to 
the geological structure of the district in which they are seated 
than the agate-industry of Oberstein. Those hills which rise 
behind the town in grotesque crags, crowned by the relics of 
baronial castles, consist of an eruptive rock which Herman 
geologists are in the habit of calling melaphyre. It is this 
rock too which is penetrated by the railway in the neighbour- 
hood of Oberstein, and has thus given rise to the cuttings and 
tunnels which the visitor passes through, whether he approach 
the town on the one side from Bingen, along the foot of the 
Hundsriick, or on the other side from Treves through the rich 
coal-field of Saarbriicken. The melaphyre has burst through 
the sandstones of this coal-field, and comes to the surface in 
several masses, the largest of which occupies a considerable 
area around Oberstein, where it is surrounded on all sides by 
Permian rocks, and is cut through by the river Nahe and its 
tributary streams. 
It would be difficult to find a word in the geologist’s vocabu- 
lary which has been more abused than Brongniart’s name 
* c melaphyre,” save perhaps our conveniently ambiguous term 
“ greenstone.” A good deal of uncertainty hangs over the 
original definition, but this has been vastly increased by the 
different ways in which the term has since been applied. A 
plagioclastic felspar is the prime constituent ; and, according 
to M. Delesse’s analysis, the felspar of the Oberstein melaphyre 
