﻿252 DR. A: HOLMES OK THE TERTIARY [vol. lxxii, 



Heulandite (CaAl 2 Si 6 0 1G 5H o 0), with its bright cleavage- 

 surfaces and pearly lustre, occurs in great abundance throughout 

 the area, from Mokambo Bay to the Sanhuti River. Cleavage- 

 flakes give good biaxial figures, which are optically positive. The 

 refractive index is T5 and the extinction -angle practically straight. 

 Heulandite is frequently followed (for instance in 125 a) by 

 downy sheaves and radiating granules of yellow laumontite 

 (CaAl. 1 Si 4 O r ,4H 2 0). Individual crystals are so small that a quali- 

 tative analysis was necessary to complete the identification. The 

 mineral gelatinizes with hydrochloric acid, and contains alumina, 

 iron-oxide, lime, and much water. The refractive index averages 

 1-52. 



In one specimen divergent aggregates of stilbite were found 

 on heulandite. and were distinguished from the latter by the larger 

 extinction-angle, and by the fact that cleavage-flakes do not show 

 a complete biaxial figure. All the zeolites -were carefully tested 

 for soda, but of the three already mentioned it was detected only 

 in stilbite; natrolite (Na,Al.,Si 3 0 10 -(-2H 0 0), on the other hand, 

 is free from lime. Slender needle-like crystals of this mineral are 

 found on heulandite, and in No. 136 b radiating sheaves of natrolite 

 complete the filling of vesicles already lined with heulandite. The 

 refractive index is 1'47, and the extinction is straight. 



Heulandite is frequently replaced by opal, chalcedony, and 

 quartz, and in some sections (as, for example, in 136, PI. XXI, 

 fig. 4) it is clear that the change began at the walls of the 

 amygdales and developed inwards. There is no evidence that the 

 other zeolites have been similarly replaced, but both laumontite 

 and natrolite are sometimes succeeded by chalcedony and quartz. 

 Calcite is exceedingly rare, and it has only been found in one 

 specimen (No. 136 b), in which it was deposited in minute crystals 

 on quartz. Gypsum is also rare, but was met with in some 

 of the sections in narrow veins cutting through everything from 

 heulandite to quartz. The order of deposition in the vesicles may 

 now be stated for a number of typical cases. Chlorite is omitted, 

 as it is liable to appear at any or every stage up to quartz. 



No. 125 a. 

 Heulandite. 

 Laumontite. 



Chalcedony. 



Quartz. 



Gypsum. 



No. 133. 

 Heulandite. 

 Stilbite. 



No. 136 a. 

 Heulandite. 



Opal. 



No. 136 b. 

 Heulandite. 

 Natrolite, 



Chalcedony. 



Quartz. 



Calcite. 



No. 125 b. 



Opal. 



Chalcedony. 



Quartz. 



Gypsum. 



The most striking feature of this suite of minerals is the pre- 

 dominance of lime and silica, and the rarity of alkali and carbonate 

 minerals. The sequence in No. 136 b is significant, for natrolite 

 free from lime follows heulandite free from soda, while finally 

 calcite appears. In the Sanhuti basalt (p. 241). thomsonite rich 

 in soda is followed by calcite. In the phonolite from the Sanhuti 

 district, natrolite is followed by calcite. There is thus a notable 



