132 Middle m i ss .- the geology of idar state. 



Afc the first locality the rock is a narrow dyke. 1 foot wide, 

 intrusive along the bedding-planes of the 



Olivine clolerite. i XT \ r. ,...,,,. ,„ , ,. 



oale-gneiss series, specimen No. .f r ; , (12 t96, PL lb, 



fig. 1). In the hand-specimen it is a dark grey, almost compact-looking 

 rock. Under the microscope it is fine-grained and almost, or 

 altogether, holo-crystalline, consisting of large and small lath-shaped 

 plagioclase crystals with the extinction angles of labradorite, augite 

 in clusters of hypidiomorphic grains, partly ophitic as regards the 

 felspar, and of pale violet-brown colours, slightly pleoehroic from 

 pinkish brown to yellowish brown. It is here and there seen to 

 be changing to carbonates which also fill in a few rounded gas pores 

 and replace some possible traces of glassy base. Olivine, in rather 

 large roughly idiomorphic .mains, is not very numerous and is also 

 partly changing to carbonates. There is some iron ore. The rock 

 is therefore an olivine dolerite or perhaps basalt. 



At the locality $ mile N.E. of Hera there is a group of low hills 

 of apparently the same rock, but specimens from this locality have 

 apparently not been collected. Besides this the basic dyke-rock 

 crops out in a desultory way through the alluvium to the east, 

 south, and south-west of Rera for half a mile or so. 



At 1 mile N.N.W. of Kawa village the presumed continuation 

 Kawa of this dyke, No. ^ (12497, PL 16, fig. 2 nat. 



light fig. 3 crossed nicols) has widened out to 

 100 yards or so, become coarser grained, and, besides the 

 labradorite (with symmetrical extinctions of the albite twin 

 lamellae up to 31°) and olivine and iron ores as before, 

 it contains a fair amount of biotitc with which the iron ore is 

 associated. The pyroxene has become less deeply coloured, and 

 is full of brown diallagic interpositions. It is very conspicuously 

 ophitic over large areas in the slide (12197). The rock is typically 

 gabbro-like in the arrangement of the minerals. At the south 

 end of the Kawa hill the thickness has increased to perhaps 200 

 yards, but the composition and structure of 4 -<£ 4 (12198) remains 

 the same as that of 4 a A (12197), whilst a finer-grained variety from 

 the same locality, 4 a r, 3 (12199), shows the pyroxene hugely re- 

 placed by uralitic hornblende, and there is no olivine. This dyke 

 is intrusive through the Idar granite, and has given rise to a hybrid 

 contact variety which will presently be described. The Kawa 

 dyke from this description is clearly an olivine dolerite, or gabbro, 

 with biotite. 



