3000 ROCK WAFERS.

Auckland Star, Volume LXVI, Issue 307, 28 December 1935, Page 7

3000 ROCK WAFERS.

GEOLOGIST AT WORK.

N.Z. SPECIMENS GATHERED.

MICROSCOPIC SAMPLES.

(By Telegraph.—Special to "Star.")

WELLINGTON, this day.

The real evidence of 40 years' experience of rocks has been crowded by Dr. P. Marshall, of Wellington, into a space no larger than his hat. On the shelves of his unpretentious little laboratory in a corner of the yard of the Public Works Department are several hundred little black boxes, and in them are "wafers" of New Zealand rocks and minerals—over' 3000 specimens, sandwiched between pieces of glass for microscopic use. How are these "wafers" made? How can they be "thinned" so that they may ze sandwiched between thin pieces of glass? Dr. Marshall made two "wafers" for his interrogator, and made the proees? appear ridiculously simple. The "wafers," he explained, were required to be no thicker than one three-thousandth.of an inch. A flake of the specimen of rock required is ground laboriously by hand with wet emery powder upon a piece of plate glass until one surface is absolutely flat. The next step is to stick it fast to one of the sandwiching pieces of glass. The flake and the piece of glass are heated upon a sheet of metal over a Bunsen burner to the right temperature for a cement to take maximum effect. The cement also is heated — cement of shellac and balsam. The flat surface of the rock flake is pressed hard upon the surface of the glass with the cement between. In a few minutes, the flake and glass are stuck fast.

The scene then is changed to the back portion of the "shed," where belts and pulleys rumble as in a stonemason's

shop. A small disc revolves at high speed on a horizontal plane. Wet carborundum powder is used as the grinding agent —twice as hard and coarse as erriery powder. The top side of the flake then is laid upon the whirling disc, the operator being wary that he does not grind his finger tips. In a minute or two the flake is ground to a thinness consistent with safety for the glass to' which it is stuck. The rest of the flake is ground off by hand with emery powder upon the plate glass surface.

A good section is a fiftieth of a millimetre! The leaves of a book would need to be very thin indeed to be compressed together in less than one inch. When the flake is about that "thick," another piece of glass is cemented on top of the rock wafer, completing the sandwich for microscopic examination, study and comparative analysis.

Rocks and minerals are very difficult to identify. Examination is made of wafe'rs in polar light to reveal crystal formation by reference to architectural, mathematical and optical attributes, and elsewhere the chemical attributes are ascertained.

Besides the optical attributes in polar light another test is applied —the way in which the mineral breaks up the light between crossed - nicols. Observed directly, specimens have colour of their own; observed between crossed nicols, however, the mineral fragments break up the polar light with definite characters of light bands—variegated but individually stable "rainbows."

Articles relating to the work of Dr. Marshall appeared in the "Star" on December 20 and 20.