286 THE HISTORY OF OLIVINE ROCKS. 



Influence of the Magnesia in Basalts on the Production of Dolomite. 



Leopold Von Buch, 1 by a survey of the southern flank of the Alps, 

 was led to believe (i) that the elevation of the eastern range of the 

 Alps, since the tertiary epoch, was contemporaneous with and de- 

 pendent on the eruption of the basaltic rock termed melaphyre; (2) 

 that the dolomites of the Alps were produced from ordinary limestone 

 at the same time and with the same dependence. The line of dolo- 

 mites and melaphyres extends (interruptedly) from Bleiberg to Lake 

 Lugano ; but the presence of dolomitic limestone in other situations 

 than where melaphyre shows itself, must render inconclusive the 

 inferences drawn from their association in the Alps. At Lugano it is 

 rather near the augitic rock than in contact with it, that the limestone is 

 dolomitised. Between the dolomite and melaphyre of the peninsula 

 of Lugano, mica schist and porphyry intervene ; and on Monte 

 Argentera, the limestone which lies upon the melaphyre is not dolo- 

 mitised. De Beaumont observes that it is even rare to find the 

 dolomites near Lugano in actual contact with melaphyre. 



Yon Buch assumes, it is to gaseous eruptions accompanying the 

 basaltic eruption, that we must ascribe the alteration. But we might 

 with more reason attribute the change to the influence of waters 

 charged with magnesia, liberated during denundation from the decay 

 of augite in long periods of time. 



Peridotite. 



Rosenbusch includes pikrite, Lherzolite, olivine-rock, eulysite, and 

 Dunite under the name peridotite. 



Pikrite is usually a combination of olivine and augite, and is often 

 more or less altered into serpentine ; and according to the amount of 

 change depends the development of magnetite at the expense of pico- 

 tite, which is embedded in the olivine. The olivine is sometimes 

 embedded in the augite. The rock approximates to those varieties of 

 olivine-dolerite in which there are few crystals of plagioclase. The 

 augite frequently decomposes into a fibrous chloritic mineral. In the 

 Fichtelgebirge, where pikrites occur in many localities, titaniferous 

 iron is common, but in the pikrites of the Rhenish Uebergangsgebirge 

 it is replaced by magnetite. Hornblende and biotite occur as accessory 

 minerals. 



Olivine-Diallage Pikrite. Another rock of this group consists of 

 olivine and diallage, and may be compared to an olivine-gabbro from 

 which the felspar has disappeared. It may include magnetic iron, 

 titanic iron, and chromic iron, and as accessories, hornblende and 

 magnesia-mica. This rock is usually more or less converted into 

 serpentine. It is well seen between Bensheim and Darmstadt. 



At Schriesheim, near Heidelberg in the Odenwald, the pikrite is 

 a mixture of olivine, hornblende, and some plagioclase, with a little 



1 Ann. des Sci. Nat., torn, xviii. pi. vii. 



