NO. 14.] 



MINERALOGICAL COMPOSITION OF THE SAMPLES. 



45 



The actual difference in refraction is observed with extreme ease and 

 distinctness by the method recently described by Schroeder van der Kolk 1 , 

 namely, the employment of oblique transmitted light. If a shade be pushed 

 under the slide until it reaches the middle of the grain of sand, the latter 

 appears to have a bright edge on the side turned towards the shade, and a 

 dark edge on the opposite side if the refraction of the grain is greater than 

 that of the surrounding medium; if it is less, the two sides are reversed. In 

 practice this kind of illumination is effected most simply by inserting a ruler in 

 the space between the mirror and the slide, and at the same time endeavouring 

 to hold the ruler as near to the latter as possible, or otherwise the opposite 

 condition of light in the grains may supervene. In this manner a considerable area 

 of the preparation may be rapidly inspected, and the separate grains of felspar 

 are then easily recognisable at the moment they come in contact with the 

 edge of the shadowed part. If the grains are then examined between crossed 

 nicols, a large proportion of them will show distinct oscillatory twinning, while 

 the remainder will not show the slightest difference from the quartz grains, 

 and will here be designated orthoclase. Under this heading, however, are 

 classed all albite and oligoclase particles that have the twin lamellae lying 

 horizontally, and also grains of these minerals, that only contain a single 

 individual. They will certainly not occur, however, in any very great 

 quantity. 



Table of the Percentage of Minerals in the Lighter Parts of the Sand 

 (specific gravity less than 285). 



1 "Bijdrage tot de mineraaldeterminatie", Handelingen van het 7" Natuur- en Genees- 

 kundig Congres. 



