71 
border. The phenomenon is really resistance to transpor- 
tation of the consolidated crust by wind rather than abrasion 
or erosion of the underlying loose sand by aeolian agency. 
A further factor that plays an important part in the 
weathering of rocks in the desert was noted in the outcrops 
of garnetiferous gneiss immediately west of the shores of 
Lake Wilson. This form of weathering, the Seele der Ver- 
witterung of Schweinfurth, consists of the flaking off of the 
rock as a result of crystallisation of salt within minute fis- 
sures in the mass. Portions of the outcrops, that have been 
previously locally hardened by cementation (concretionary), 
have resisted this weathering to some extent, and consequently 
those portions project from the surface of the decomposing 
gneiss as irregular, partly serrated, ridges, the direction of 
which is usually consistent with that of an original constant 
geological feature of the rock. 
Veins, etc. — Comparatively few true fissure veins or 
lodes were noticed in the Mann Ranges. At the salt pan 
just mentioned an exposure of a ''quartz reef" occurs in com- 
bination with a coarse pegmatite (i.e., secondary quartz, in 
the intrusive). The quartz of the '"reef" is very coarsely 
crystalline, the faces of the prisms exhibiting oscillatory com- 
bination to a marked degree. The felspar of the pegmatite 
occurs as large pink idiomorphic crystals of orthoclase. The 
lode is non-metalliferous. 
A common method of formation of so-called ''quartz 
blows" in the ranges is nothing more than metamorphism 
by igneous intrusion into the bedrock, the ultimate product 
consisting of a highly altered quartz schist. The best 
example of this phenomenon was met with south-east of 
Mount Edwin. The quartzose outcrop there consists of 
three parallel ridges of metamorphic quartz schist and 
granular quartz, the planes of schistositv of the former 
being visible either as thin layers of secondary mica or the 
direct products of decomposition of the same. ' The outcrop 
trends W. 40° S., and is jointed in directions: (a) N.E., 
dipping 70° S.E., the rock being finely laminated in this 
direction, and the planes of lamination a fraction of an 
inch in thickness : (b) N.W., in well-defined, parallel planes, 
few inches apart ; (c) W. 10° N., and N. 20° W., in less 
perfect partings. This quartzitic exposure is, beyond doubt, 
a true product of contact metamorphism, and the difference 
between its strike and that of the country is explained by 
parallel outcrops of garnetiferous diorite dykes between the 
separate ridges of the formation; for these have been the 
cause of the metamorphism of the original schistose beds 
lying directly in contact with them. 
