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52 STRUCTURAL AND FIELD GEOLOGY 
have a doleritic aspect. Phenocrysts of olivine and augite are often 
more or less conspicuous. 
Leucite-basalt, dark grey to black, has for its essential constituents 
leucite, augite, and olivine. Amongst the accessory ingredients are 
nepheline, biotite, hornblende, haiiyne, apatite, magnetite, etc. The rock 
is fine-grained, and usually shows phenocrysts of augite or olivine or both. 
Little or no glassy base is present. 
There are several other rocks included in this division, amongst 
which are Wephelinite=augite+nepheline ; Leucztite=augite + leucite. 
4. ROCKS WITHOUT FELSPARS OR FELSPATHOIDS 
These are dark coloured, heavy, ultra-basic rocks—the 
silica percentage hardly averaging more than 43, while the 
specific gravity ranges between 2-7 and 3:5. 
Limburgite (J/agma-basalt) is a reddish-brown or dark grey to black 
rock, composed essentially of augite and olivine set in a glassy base. 
The rock is either fine-grained or compact, with a pitch-like lustre. 
Under the microscope it is seen to consist largely of glass, which, 
however, is sometimes so crowded with microlites of augite, magnetite, 
etc., that little or no glass may be visible. Phenocrysts of olivine and 
augite are usually conspicuous. Amongst the accessory ingredients 
are magnetite, ilmenite, biotite, hornblende, haiiyne, etc. Augitite is 
a black rock composed essentially of augite and magnetite in a glassy 
base, and resembles limburgite, from which, however, it is distinguished 
by the absence of olivine. Neither of these rocks is important so far as 
its distribution is concerned. The former takes its name from Limburg, 
near the Kaiserstuhl in Baden, and is met with in various other places in 
Germany. It occurs also in southern Sweden, Spain, the Canary 
Islands, and in Central Scotland. Augitite is found in Bohemia, Central 
France, the Canary Islands, the Cape Verd Islands, and Ireland. 
Peridotites.—These are the most basic of igneous rocks, 
and are composed mainly of olivine—hence often designated 
“olivine-rocks.” With this constituent are associated small 
proportions of one or more of the following minerals— 
picotite, chromite, augite, diallage, hornblende, biotite, 
enstatite, apatite, magnetite, ilmenite, garnet, etc. 
A number of varieties have been described, of which the following may 
be mentioned :—Dunite, composed almost wholly of olivine with a little 
picotite or chromite—occurs in the Dun Mountain, S. New Zealand; 
Picrite,a rock rich in idiomorphic olivine, with which are associated in 
varying amount pyroxene (augite, hypersthene, enstatite), hornblende, 
biotite, magnetite, ilmenite, apatite, and not infrequently a little plagio- 
clase. According to the relative predominance of augite, hornblende, 

