HORN EXPEDITION — GENERAL GEOLOGY. 45 



A second specimen (No. 9G) is a gneissoid granite fi'uni Alice Springs with 

 large poi-pliyiitic niicrocline crystals. Tiie mass of the rock scarcely exhibits 

 foliation in hand specimens, hut when examined in tlie field it is found to be 

 distinctly foliated. It consists of quartz, felspar (microcline), and twu varieties of 

 mica, biutite and niuscovite, of which the former is most abundant. 



A third specinuui (No. 139) is from a dyke intruding gneiss near the head of 

 Ellery's Creek. It consists of grey (juartz, niuscovite, and a white rather opaque 

 plagioclase exhibiting veiy fine twin lamellation parallel to O'lO, and very low 

 extinction angles. This granite resembles in its mineralogical composition, and its 

 structure, that occurring (»u the north side of Ilar't Range, which latter has 

 yielded the \ery large mica crystals. jNIany of the gneisses, especially in the 

 vicinity of intrusive masses, are found to contain epidote, sometimes in large 

 quantities. The epidote sometimes occupies small veins in the gnei.ss ; at other 

 times it is found to be present in the form of irregular grains throughout the mass 

 of the rock. 



[/>) Interme(liate group. 



Near the ( )olgarna mica-claim a Hne-grained intrusive rock occurs, which is, 

 at first sight, not unlike a basalt, though somewhat lighter in colour. In thin 

 sections it proves to be a granulitic jiyroxene diorite with much hypersthene, and 

 a fairly large quantity of magnetite and uralitic hornblende. 



((■) Basic Group. 



Between 81ip-paiiel Gap and Ellery's Creek a large number of dykes of 

 basic rocks occur intruded amongst the gneisses, etc. The strike of these 

 dykes is usually N.W. and S.E., some of them Ijeing traceahh^ on the surface for 

 long distances. One dyke was obser\ed to be as much as GOO feet in width, 

 the rock of which it is composed being an olivine dolerite. The basic rock of one 

 of these dykes proved to be a gabbro (No. 109) consisting of diallage, a plagioclase 

 with an extinction angle about that of labradorite, and magnetite in large grains. 

 Another of those Ijasic rocks (Nos. 19') and 179) appears to Ik- an olivine dolerite, 

 consisting of plagioclase, augite, a considerable ([uantity of undecomposed oli\ine, 

 magnetite, and a few grains of hypersthene. ()phitic structure is well developed, 

 in which large irregular masses of augite enclose a number of felspar crystals. 

 Another rock (No. 64) is a diabasic dolei'ite, in which much of the felspar and 

 augite is decompo.sed resulting in the production of epidot(^, chlorite, fibrous horn- 

 blende anil, perhaps, saussurite. 



