HISTORY OF GABBRO. 227 



to the N.E. of the Havallah range, where quartz forms a sort of 

 colourless ground mass in which the hornblende is distributed. 

 Larger crystals of quartz and hornblende occur, and there is some 

 brown mica. 



Gabbro. 



Gabbro consists essentially of plagioclase and diallage. Olivine is 

 rarely an essential constituent. Magnetic iron and titanic iron occur, 

 apatite is abundant ; and many gabbros contain hornblende, rhombic 

 pyroxene, brown biotite and quartz as accessories. Gabbros in 

 Guernsey contain hornblende as a chief constituent, but it is probably 

 a decomposition product. Some gabbros approximate to the diabase 

 variety of dolerite. Other gabbros have the plagioclase converted into 

 saussurite, and the diallage more or less replaced by smaragdite. 



The plagioclase is either labradorite or anorthite, the latter species 

 being present when olivine occurs. Orthoclase has only been 

 recorded in the gabbros of Monzoni and Yal di Susa. 



The diallage often tills up the interspaces between the plagioclase ; 

 it aggregates round the olivine when olivine is present. At Volpers- 

 dorf, diallage and rhombic pyroxene are intimately associated. The 

 pyroxene may be hypersthene, enstatite, or bronzite. This inter- 

 growth of pyroxene parallels the intergrowth of felspars. Parallel 

 interlamiriation of crystals of diallage or hornblende with plagioclase 

 occurs. The hornblende is sometimes brown, sometimes green, and 

 sometimes results from the decomposition of diallage. The diallage 

 may be nearly colourless, greenish, or brownish. Biotite is the con- 

 stituent next in importance after diallage ; it is abundant in the rock 

 at Waldheim in Saxony and Todtmoos in the Southern Schwarzwald. 

 When enstatite and bronzite occur, they decompose and form bastite. 



In olivine gabbros, sometimes olivine, sometimes plagioclase, pre- 

 dominates. When the quantity of felspar is small, the development 

 of serpentine is greatest. Occasionally diallage is absent. The 

 gabbro of Penig is rich in hypersthene, and has little olivine, and so 

 resembles that of Loch Scavaig and the Cuchullin Hills. At Haus- 

 dorf the gabbro is free from olivine. A rock of this kind at Harzburg 

 is rich in biotite, and contains augite and quartz. 



The silica in gabbro usually varies between 43 and 50 per cent., 

 alumina 13 to 20 pef cent., with an average of 10 per cent, of iron, 

 10 per cent, of lime, and 10 per cent, of magnesia soda and potash. 



Geographical Distribution of Gabbro. In Bohemia, near Eons- 

 berg, the gabbro contains crystals of diallage several inches in 

 diameter, which are surrounded with crystals of hornblende. In the 

 Vosges the gabbro is intrusive, and is sometimes converted into 

 serpentine. In Italy it is seen between Genoa and Savona, and is 

 associated with serpentine in the coasts south of Livorno. In Corn- 

 wall it is associated with the serpentine of the Lizard. 



Murchison describes hypersthenite in the Stanner Rocks, near 

 Kingston, by Old Radnor, where it is intrusive in Wenlcck limestone. 



