1907-1908.; 



93 



Stilbite, and Thomsonite were described in detail, numerous 

 specimens being exhibited in illustration, and their mode of 

 occurrence in the Tertiary Basalts, at various localities, was also 

 described. 



The main portion of this paper was devoted to the geogony 



of Zeolites in Basalt, as revealed by a study of many specimens 



from various localities in Co. Antrim, and the following facts were 



pointed out as generally typical of the district : — 



i. The Zeolites occur in spaces in the Basalt varying from i mm. 



in size to as much as 10 cm. The cavities, usually called 



vesicles, show an evolution in their shape, from mere 



interstices in the ground of the rock to true cavities. Various 



types in this development are as follows : — interstitial spaces, 



small drusy spaces, drusy spaces connected by narrow 



interstitial spaces, true cavities divided into cells by filaments 



and shreds of the Basalt, irregularly-shaped cavities, and 



finally, more or less spherically-shaped cavities. 



2. The Zeolites have been, in most cases, deposited in a definite 



sequence, and when a number of Zeolites are found crystal- 

 lized together, such a sequence is quite evident. The most 

 common sequence in Co. Antrim is as follows : — Natrolite 

 (which is almost invariably the first Zeolite to be deposited), 

 Chabazite, Analcime, Stilbite, or sometimes Apophyllite. 

 Another sequence commences with Thomsonite, followed by 

 Chabazite and Analcime. When Calcite occurs in a cavity 

 with Zeolites it is generally deposited before any of the Zeolites. 



3. The cavities containing Zeolites are generally lined or coated 

 with a layer of a " green earth," such as Saponite, which varies 

 in thickness from a mere film to as much as 1 cm. This 

 material resembles closely in composition and structure, and 

 appears to be identical with, the interstitial glassy material of 

 the Basaltic ground-mass. Thus the complete sequence 

 sometimes observed in Zeolitic deposits is (1) Green glassy 

 material (2) Calcite (3) Zeolites in sequence. 



