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Parr IL § vi] CRYSTALLINE ROCKS—MASSIVE. 151 
one of the products of the active and extinct volcanoes of 
Southern Italy. Leucite-basalt is to outward appearance 
quite like true basalt, and occurs under similar conditions, but is 
less widely distributed than even nepheline-basalt. Under the 
microscope it presents a finely crystalline structure with little trace 
of any amorphous base, and abundant minute sections of the 
characteristic leucite. This rock occurs among the extinct volcanic 
cones of the Hifel, in the Thuringer Wald, and in the Italian volcanic 
districts (Albano, Capo di Bove). Leucite-rocks, so far as known, 
occur only among later Tertiary and recent volcanic products. 
(4.) Olivine Rocks. 
This division embraces a series of crystalline rocks composed 
essentially of olivine, with usually one or two other magnesian 
silicates. Rocks of this type have been classed by Rosenbusch 
under the general name of Perzdotites. The following are the more 
important species :— 
Pikrite,arock rich in olivine, usually more or less serpentinized, 
with augite, magnetite, or ilmenite, and a little brown biotite, horn- 
blende, or apatite; eulysite, a mixture of olivine, augite, and red 
garnet; garnet-olivine-rock, composed of olivine, diallage, and 
garnet; olivine-enstatite-rock consisting of olivine and enstatite 
(bronzite or hypersthene) with magnetite or chromite; lheraolite,a 
mixture of olivine, pyroxene, picotite, and usually some magnetite’; 
dunite, a mixture of olivine and chromite, found with serpentine ; 
limburgite, composed of crystals of olivine, augite, and magnetite, 
in a base more or less vitréous. 
One of the most remarkable features about these rocks is their 
frequent association with serpentine and their tendency to pass into 
that rock. There can indeed be no doubt that, as Tschermak first 
pointed out, many serpentines were once olivine rocks. 
(5.) Serpentine Rocks. 
_ Under this name may be included rocks which, whatever may 
have been their original character and composition, now consist 
mainly or wholly of serpentine. As already stated, olivine readily 
passes into the condition of serpentine, and many serpentine rocks 
originally consisted principally of olivine. This mineral may be 
changed into serpentine, while the other minerals remain nearly 
unaffected, as is admirably seen in pikrite. If varieties due to 
different phases of alteration were judged worthy of separate desig- 
nation, each member of the olivine rocks might of course have a 
conceivable or actual representative among the serpentine series. 
But, without attempting this minuteness of classification, we may 
- with advantage treat by itself, as deserving special notice, the massive 
1 Bonney, Geol. Mag. iv. 2nd ger. p. 59. 
