HORN EXPEDITION— GENERAL GEOLOGY. 41 



crust-pi'essuro, j,'ivo tho rocks n Ijfddod appoaranco. Whoa, liowoxor, wc trace 

 tlipse pseiido-sti'ata along the surface in tlie direction of their striice, we (lad tliafc 

 niaay of theia are not pei-sisteat even for sliort distances, but pinch out so as to 

 have the sliape of lenticular masses. AVhen therefoi-e we speak of the direction of 

 stiike and angles of diji of these Pre-Caaibi-iaa rocks, such terais auist be considered 

 to apply to the planes of foliation. Rocks exhibiting all degrees of foliation 

 appear to be present from the coar.se-banded gneissic granite from Alice Springs, 

 in wliich the foliation is scarcely noticeable in hand specimens, to such finely- 

 banded gneiss as tiiat which is largely developed near the head of Ellery's Creek. 

 In the Belt Range the dip of the foliation planes increases from 40° on tlio .south 

 to G0° and 70" on the north side of the range. The foliation planes of the gneiss 

 near Slip-panel Cap, al)out sixty miles west of Alice Springs, strike N.K. .and S.W., 

 and dip at high angles to N. W. Further east, on the south side of Mount Conway, 

 the strike was found to hav(> changed to E.N.E. and W.S.W., the dip Iieing al)out 

 70° S.S.E. Still further to the east, between the last-mentioned place and Alice 

 Springs, the strike was observed to l)e E.S.E. and W.N.W., while the dip jiad 

 decreased to 37° S.S.W. To the north and south of Alice Springs the foli.ition 

 planes of the gneiss dip at 40° to 50° S.W., the strike being N.W. and S.E. 



In the McDonnell Ranges, to the east of Alice Springs, the strike was the 

 same as that noted north and south of Alice Springs, but the dip had decreased to 

 2-5° S.W. On the Ij.Luks of Jessie Creek, ten miles east of Alice Si)rings, the strike 

 of tlie gneiss forming the low hills there was N.E. and S.W., and the diji about 

 40° N.W. In the Hart Rangt> tiie planes of foliation were found to be well 

 marked; their strike varies from \\'.S.W. and I].N.E. to S.W. and N.E., and the 

 dip ol)ser\-ed varied from 50' to HO in a direction varying from N.N.W. to N.^\^ 

 Lastly, in the Str.angway Range, at Winnecke's Depot, the gneiss was seen to dip 

 at 45° N. 10" E. Theie is a remarkable absence of evidence of tlu^ original planes 

 of f(jliation hasiiig Ijeen subjected to any subsequent contortion, on either a l;irge 

 or a small scale, a fact which seems to imply that the dii-ection of the prt\ssui'e was 

 constant ff>r a prolonged time, and that the rock-masses were able to find lelief 

 from the stresses to which they were subjected Ijy (he de\e]opment of gliding 

 planes and sheer planes. 



{(') Material Geology. 



The rocks of this group m;iy, for purposes of description, ije roughly cLissilied 

 as follows : — 



1. Those whose sedimentary origin is evident. 



