AGATES AND AGATE-WORKING. 
25 
appears to be labradorite.* Specimens from the railway- 
station contain a plagioclase of blood-red colour, due to the 
presence of lamellae of ferric oxide.f Microscopic research has 
shown that in many melaphyres the plagioclase is associated 
with an orthoclase. Formerly it was supposed that the rock 
was destitute of olivine, and in this respect differed markedly 
from basalt ; it has, however, been found of late years that olivine 
is frequently present, and indeed it is difficult to separate some 
melaphyres, when fresh and unweathered, from true felspar- 
basalts. Augite, however, is not so constant a constituent of 
melaphyre as was formerly supposed ; in some cases the augite 
appears to be transformed into a chloritic mineral, and indeed 
much of the Oberstein rock has a greenish tint. Magnetite is 
always present, as in so many other eruptive rocks. 
Some varieties of melaphyre are compact in texture, others 
porphyritic, and others again amygdaloidal. All these varieties 
are to be collected in the neighbourhood of Oberstein, but it is 
only the last named that is of interest for our present purpose. 
Just as the carbonic anhydride disengaged during fermentation 
imparts a cellular character to the dough, which is retained in 
the bread ; so, while the palaeozoic lava, which we call mela- 
phyre, was still plastic, bubbles of gas or of steam were dis- 
engaged, and have left their impress in the rock, the molten 
matter having been sufficiently tough to prevent collapse of the 
walls. Although the normal form of these bubbles would be 
more or less globular, or probably pear-shaped, with the narrow 
«nd downwards, it has generally happened that, during the flow 
of the lava, the cavities have been drawn out in the direction 
of the current, so as to form elongated rather flattened cavities, 
resembling an almond in shape, whence the common name 
Amygdaloid or Mandelstein. These vesicles are in some cases 
empty ; but usually they are filled to a greater or less extent 
with mineral matter, which has been deposited in them by 
chemical changes occurring in the rock subsequent to its for- 
mation. In many amygdaloidal rocks the mineral is merely 
carbonate of calcium, as may frequently be seen in our well- 
known Derbyshire “ toadstones,” which are melaphyres inter- 
bedded in the Carboniferous Limestone. But the cavities in 
the amygdaloidal rocks of Oberstein are for the most part filled 
with silica in some of its Protean forms. As you pass along 
cuttings by the road-side you may see the rock charged in some 
places with myriads of little chalcedonic nodules, or rudimen- 
tary agates, which look like so many fossil almonds. Indeed, 
* Delesse : “ Sur le Porphyre Amygdaloide d'Oberstein ” (“ Ann. des 
Mines/’ [4], xvi. p. 511.) 
t Zirkel : u Die mikr. Beschaff. d. Min. u. Gesteine,” p. 414. 
