8PRINGS AND THOSE OBSERVED IN AMYGDALOIDS ETC. 75 



results of this preliminary examination being contained in Note 2 

 (p. 83). 



In the altered bricks the cavities, instead of remaining empty, are 

 irregularly filled with colourless and transparent substances which 

 are without doubt due to the slow and prolonged action of the heated 

 water to which they have been subjected (PI. IV.). 



The following are the principal substances that I have observed in 

 the pores of the bricks : — 



1. Chabasite in cube-like rhombohedra without cleavages, about 

 0-08 m.m. in the side. 



2. Prisms disseminated through the paste, which are possibly 

 christianite * (phillipsite). 



3. Mesotype, recognizable by the crystalline forms,which are prisms 

 with basal terminations and grouped radially (fig. 3). They have a 

 decided action upon polarized light, and when subjected to the action 

 of acid leave as a residue prisms of the same form, which have no 

 action upon polarized light, and are probably siliceous skeletons of the 

 original crystals; the leading dimensions are 0-1 m.m. long and 0*02 

 m.m. broad. This method of occurrence is exactly similar to that 

 observed in the pores of certain basalts, a specimen from Donnersberg 

 in Bohemia offering a striking example of this resemblance. 



4. Substances in mamillated concretions concentrically banded, 

 which are not attacked by acids, scratch felspar, and have no action 

 on polarized light. These are probably opal, of the variety known 

 as hyalite. 



5. Hexagonal plates with rounded angles aggregated in the manner 

 usually observed in tridymite, to which species they probably belong, 

 as they are insoluble in acids. 



6. Pibrous radiated spherules, having a powerful action upon 

 polarized light and giving a fixed black cross when rotated between 

 crossed nicols, or the optical properties characteristic of chalcedony, 

 which substance they further resemble in being harder than felspar 

 and unaffected by acids. Occasionally these globules are arranged 

 along the walls of the cavities in the form of contiguous hemispheres, 

 their maximum diameter being 0*02 m.m. 



7. In addition to the fibrous spherules, others of smaller size are 

 found distributed through a colourless transparent and optically in- 

 active substance, which is opal. These globules have a feeble 

 action upon polarized light, like that of gummy substances, and are 

 often arranged along the sides of a cavity, forming a kind of border, in 

 a manner exactly similar to that observed in certain rhyolites, as, for 

 example, those of Nisiros, and of Telkybania in Hungary. 



8. Upon one specimen globules resembling those last noticed have 

 been observed, which have a yellowish tint, and are attacked by 

 nitric acid at a temperature as low as 30° C, which characters are 

 analogous to those of the globules of similar appearance usually found 

 in palagonite. 



9. Calcite is often found completely filling the cavities, with the 



* Christianite is used throughout as synonymous with phillipsite, and is not 

 to be mistaken for the variety of anorthite known by the same name. 



