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THE GEM SANDS OF ENCOUNTER BAY. 



By R. Grenfell Thomas. 

 [Read September 13, 1923.] 



The beach sand at several points in the vicinity of Encounter Bay has long 

 been known to yield a so-called "gem sand" where concentrated by wave action. 



The underlying rock formations comprise intrusive granite and a series of 

 thermally metamorphosed sedimentary rocks, which are taken to be of Pre- 

 Cambrian age. These older rocks are overlain at intervals by irregular and 

 discontinuous patches of glacial drift of Permo-Carboniferous age. 



As this sand is representative of the heavy mineral content of the area, its 

 composition may be expected to furnish interesting evidence of the alterations 

 occasioned by thermal contact metamorphism in this region. It was primarily 

 with a view to ascertaining the mineral composition of the sand that the follow- 

 ing investigation was undertaken. 



Mode of Occurrence. 

 The gem sand, which is conspicuous by its red colour, occurs as irregular 

 thin-bedded sheets on the beaches in question near high-water mark. The pre- 

 sence of concentrated deposits of the sand in any one area is not necessarily 

 dependent on the immediate proximity of rocks in which the constituent minerals 

 occur in situ, but is largely influenced by prevailing currents, slope of beach, 

 and general coastal configuration. Although concentration by wave action is 

 the chief factor influencing the deposition of the sand, the effect is often amplified 

 by the selective action of winds on the sand dunes, whereby the lighter grains 

 are carried away, leaving a residue of higher specific gravity. 



Localities. 



The chief areas of deposition in Encounter Bay can be grouped under two 

 headings: (1) At Rosetta Head; (2) at Middleton Beach. For purposes of 

 comparison it is convenient to divide the Rosetta Head deposits into two classes : 

 — (a) Those on the west side of Rosetta Head, or the "Petrel Cove" type ; 

 (b) those on the east side of Rosetta Head, or the "Blufif Bay" type. At these 

 localities the gem sand is present in greater or less quantity throughout the year, 

 although the concentrated deposits are often temporarily disseminated by heavy 

 seas. 



Collection of Samples. 



Representative samples of sand from the localities mentioned were collected 

 and were each concentrated in the field, by "panning," to a well-defined stage 

 immediately preceding a loss of the heavier constituents. The material thus 

 eliminated was found to consist of quartz and shell fragments, and contained 

 none of the heavier minerals which it was desired to examine. In this way 

 samples, weighing about 10 lbs., after concentration, were obtained from each 

 locality. These samples may be regarded as thoroughly representative of the 

 several deposits, and thus afiforded standards for comparison of the different 

 types. 



Mechanical Analysis. 



An approximate determination of the average size of the grains in each 

 sample of sand was made by using a micrometer eyepiece fitted to the microscope. 



This method gave the following results : — Diameter in millimetres. 



Petrel Cove. Bluff Bay. Middleton. 

 Average diameter of largest grains . . 1 mm. 5 mm. 5 mm. 



The majority of the grains range from .. '5-3 mm. ^-^ mm. '2-1 mm. 



