no. 1726. 



ON SAND-BARITES /7,'0 1/ EGYPT— POGUE. 



23 



2. Slide prepared at right angles to a penetration twin, a micro- 

 photograph of which is reproduced in the accompanying figure. The 

 quartz presents the same features as in case 1. The barite, however, 

 shows the perfect c or basal cleavage, dividing the mineral into long 

 strips, with the less prominent m cleavages at right angles. There 

 are two individuals, each of which extinguishes in polarized light as 

 a unit, and has the same orientation of cleavage throughout. The 

 extinction is parallel to the c cleavage, and the angle measured 

 between the twinned parts is 30°. 



Photomk rograph of a thin slice of sand-baeite, cut at right angles to a penetration twin 

 "i two tabular individuals crossing at an angle of 30°. Polarized light; magnification 

 aroi't 'l'i diameters. Rounded and slightly angular quartz grains are abundantly inter- 

 spersed IN A GROUND OF REGULARLY ORIENTED BARITE. THE INDIVIDUAL To THE RIGHT IS 

 NEARLY EXTINGUISHED, WHILE THE ONE To THE LEFT IS BRIGHTLY ILLUMINATED. THE BASAL 

 CLEAVAGE MAY HE SEEN IN EACH. 



3. Cross-section of the concretionary specimen shown in fig. 1.5. 

 In this the barite is without regular orientation of cleavage, and its 

 extinction is undulatory. At no time during the revolution of the 

 stage does the baritic matrix become dark. This example differs 

 markedly, therefore, from the other instances in that the barite pos- 

 sesses no continuity of arrangement. 



Chemical. — The percentage of sand in different specimens was found 

 to vary. The limits are 44 and 53 per cent sand, with values between 



