76 PEOF. A. BAUBLE ON INCRUSTATIONS BY THERMAL 



form of the primary rhombohedron, reproducing an arrangement 

 found in the basalt previously noticed from Donnersberg. In other 

 cases it is intimately associated with opal and affects the form of an 

 acute rhombohedron. 



The different substances noticed above occur as partially or totally 

 filling microscopic cavities of different forms, these being sometimes 

 rounded, like the larger vesicles, and sometimes elongated into fine 

 cracks. Usually the concretionary substances penetrate the substance 

 of the brick to a certain depth from the walls of the cavity, forming 

 a kind of penumbra. 



The arrangement of these different substances, which I have ex- 

 amined with the assistance of M. Fouque, are represented in the 

 drawings (PL IV.), for which I am indebted to the skill of M. Thoulet. 



The presence of chalcedonic quartz in association with opal is a 

 fact worthy of attention, as proving that silica may be deposited in 

 the anhydrous form, although the temperature of the medium in which 

 the separation took place did not exceed 70° C. The production of 

 quartz at such a low temperature has not, I believe, been previously 

 noticed. 



By chemical examination I have not less certainly proved that the 

 different minerals are the results of the introduction of foreign sub- 

 stances into the body of the brick. The penetration is nowhere uni- 

 form, and may vary very considerably in the same fragment from the 

 external surface to the interior. The former part is usually compact 

 and contains up to 8 per cent, of water, so that it decrepitates violently 

 when heated to redness, while the central and more porous part con- 

 tains only 2 or 3 per cent, of water. There is a similar difference in the 

 proportion of lime and potash contained in the two parts, these bases 

 being found in larger quantity in the compact than in the porous 

 bricks. The compact character of the outer portion of the bricks 

 is therefore due to impregnation with hydrated silicates of lime and 

 potash, and probably also of alumina, such as are found in the zeolitic 

 family of minerals. 



When finely powdered and heated with boiling distilled water, the 

 bricks give a solution of silicate of potash having an alkaline reaction. 

 A complete analysis, the results of which are given in Note 3 (pp. 83, 

 84), shows that the amount of zeolitic substance added to the bricks 

 is from 13 to 14 per cent, of their total weight. 



II. The contemporary production of Zeolites and their associated 

 minerals furnishes an experimental demonstration of the mode of 

 formation of the same minerals in Amygdaloids and many other 

 classes of altered volcanic rocks. 



The brickwork altered in the manner described in the preceding 

 part, and Amygdaloids, as well as many other altered volcanic rocks, 

 have many points of resemblance in the nature of the minerals 

 developed by such changes. This similarity, or it may even be 

 called identity, applies not only to the portions visible to the naked 

 eye (zeolites, opal, aragonite, and other compounds), but extends to 



