258 



Solid inclusions often plentiful and occasionally take the form of acicular 

 rods arranged either in "sagenite fashion" or in a confused felt-like swarm, the 

 habit and optical properties corresponding to sillimanite. 



Owing to the perfect (100) cleavage the plates are usually lying on this 

 face, and hence show the almost normal emergence of the acute bisectrix, thus 

 giving good biaxial figures when examined in convergent light. The optic 

 axial angle is large and the optic sign negative. The specific gravity is 362. 



RuTiLE. — Common in the non-magnetic portion of the sand as abraded 

 rods and round grains. The Petrel Cove sand often shows good euhedral forms 

 with striated prism faces ; geniculate twins also occur. The colour is generally 

 very dark but varies considerably; the Middleton sand shows the lightest reds. 

 By reflected light the grains show the typical semi-metallic lustre. The pleo- 

 chroism, when observable, is slight. Refractive index extremely high, birefring- 

 ence also very high, extinction straight. 



A weak cleavage (110) is sometimes noticeable. Inclusions rare. The 

 specific gravity is 4 35. 



Zircon. — Abundant in the non-magnetic group. Almost invariably in 

 abraded anhedral grains, traces of crystal form being very rare. The grains 

 are always perfectly colourless. Refractive index very high, birefringence also 

 very high. Straight extinction shown. Inclusions very rare. The specific 

 gravity is 471. 



The Origin of the Component Minerals. 



As regards the origin of the various minerals occurring in the sand, it 

 appears most probable that the majority of the constituents are derived from the 

 regions of contact thermal metamorphism produced by granitic intrusions of the 

 Pre-Cambrian sedimentary series which occurs in this area. 



Some of the constituents, such as staurolite and kyanite, are certainly not 

 derived from the igneous rocks, and although several of the other minerals such 

 as rutile, zircon, and garnet are often of magmatic habit, they have not been 

 observed to be common constituents of the igneous rocks of this region, which 

 have been petrologically examined by W. R. Browne '^^^ and others. There can, 

 however, be little doubt that, in some instances, the mineralizing influence of 

 the intrusive granite has played an important part in their formation. 



That the constituents of the sand are genetically connected with the meta- 

 morphic aureoles is further supported by the fact that those sands occurring 

 furthest away from the contact zones invariably show the greatest abrasion 

 and rounding of the grains, while those adjacent to the contact zones generally 

 contain larger and more angular grains which are often euhedral. Thus the 

 Petrel Cove deposits, occurring in the midst of a strongly metamorphosed region, 

 show considerably larger and less abraded grains than the Middleton sand, which 

 appears to travel along the coast from the contact zones at Port Elliot, a distance 

 of about two miles. 



Smnmary. 



The gem sand of Encounter Bay contains the following minerals : — 

 Magnetite, ilmenite, almandine garnet, staurolite, green spinel, tourmaline, kyanite, 

 rutile, and zircon. The minerals being of relatively high specific gravity 

 are concentrated on the sea beach by wave action. The component minerals 

 appear to be chiefly derived from the metamorphic aureoles resulting from the 

 intrusion of an acid magma into a Pre-Cambrian sedimentary series. The 

 sands from various localities in Encounter Bay contain the same set of minerals 

 but the relative proportions of the minerals are not constant throughout the area. 

 Geological Laboratory, Adelaide University, 

 August 9, 1923. 



(3) Vide "The Igneous Rocks of Encounter Bay," by W. R. Browne, Trans. Roy. Soc. 

 S. Austr., xliv., p. 1. 



