SPRINGS AND THOSE OBSERVED IN AMYGDALOIDS ETC. 83 



normal state, consisted essentially of olivine. The hydration of perlitic 

 rocks, and the hydrated concretions incrusting their cavities, may 

 also be due to analogous causes. These latter rocks may be con- 

 sidered as results of the more or less advanced effects of alteration 

 by aqueous agency. 



Note 1. — It is necessary to add that these modifications in the 

 nature of the brick have not been uniformly effected. It is not 

 uncommon to find in the same specimen a friable and earthy in- 

 terior combined with a compact and altered exterior ; but however 

 irregular the fragment may be, the change has gone on from the 

 surface to a tolerably uniform depth, and in concentric bands ana- 

 logous to the structure observed in nodules of jasper. This latter 

 circumstance shows very plainly that the compact parts of the frag- 

 ments are the result of modifications to which they have been sub- 

 jected since they were imbedded in the concrete. 



Note 2. — This brick is composed of an amorphous transparent sub- 

 stance, colourless or of a pale yellow tint, with innumerable cavities, 

 through which are disseminated granules of a ferruginous silicate of 

 a bright red colour, due to ferric oxide. Grains of quartz are also 

 common, generally broken and irregular in outline, as well as particles 

 of quartzose sandstone. A single crystal of triclinic felspar has been 

 seen, besides numerous small colourless laminae, which darken under 

 crossed Nicols in the direction of the longer axes, and which are pro- 

 bably crystals of mica seen in section. The quartz contains numerous 

 cavities ; but none of these contain liquids, which may be due to the 

 high temperature to which the brick has been exposed in the kiln. 

 A fluxion structure is tolerably constant, as evidenced by the 

 alignment of the red granules, the quartz grains, and the scales of 

 mica, which follow the same general direction. 



Note 3. Chemical Examination. — The substance of the bricks, 

 averaged by sampling several specimens which were apparently 

 compact when seen by the naked eye, was subjected to analysis by 

 M. Eouque, with the following results as obtained from two samples. 



Bride A. This is of a uniform red substance, excepting a few 

 interspersed grains of quartz. It lost 4*32 per cent, in weight by 

 heating in a sand bath to 120° C. ; when calcined at a bright red heat 

 a further loss was experienced of 5*22 per cent. There was only a very 

 slight effervescence with acids. 



When heated with boiling distilled water, 10 grammes of the 

 finely powdered material gave an alkaline solution, which neutralized 

 8 milligrammes of monohydrated sulphuric acid, contained only 

 traces of alkaline carbonates, and was free from lime. "When 

 acidified and evaporated to dryness, a residue insoluble in acids was 

 obtained, proving the soluble alkaline constituent to be a silicate of 

 potash. 



Brick B. — This was generally similar in physical character to sample 

 A„ The loss of weight was 5-5 per cent, at 120° C, and an addi- 

 tional 6 per cent, by calcination. It contained no carbonates ; and 

 the solution extracted by water neutralized 9 milligrammes of sulphu- 

 ric acid. Another portion, weighing 10 grammes, was treated for 15 



q2 



