306 Dr Brewster on the Structure of Haytorite. 



to be seen whether this deviation is most common in crystallized 

 minerals, which have either no cleavage or an imperfect one, 

 and to what extent it can go without affecting the external form 

 of the crystal. When we consider what curious composite 

 structures exist in apophyllite, analcime, amethyst, chabasie, 

 mesolite, &c. &c. without any corresponding indication of it in 

 their external crystallization, the anomalous structure of Hay- 

 torite will cease to excite our astonishment. I conceive indeed 

 that it may belong to the same class of facts, with this differ- 

 ence only, that, in the case of the former, the combined indivi- 

 duals have a perceptible magnitude, whereas in the Haytorite 

 they are minute particles or granular crystals. 



In order to illustrate this view of the matter, which is pro- 

 posed merely as an hypothesis, let us suppose a compound 

 crystal such as the Sulphato-tri-carbonate of lead, which, as 

 Mr Haidinger has proved, is composed of three individual 

 crystals forming what appears to be a regular rhombohedron. 

 If we expose this crystal to polarized light, we shall see that 

 the axes of double refraction, or of crystallization, are lying in 

 three different directions. If the size of the crystal is con- 

 ceived to be reduced so that the three individual crystals be- 

 come small grains, and if we also conceive that each of these 

 grains unites with another grain according to the same law of 

 composition, the crystallization filling up any vacuities that 

 may be left, we shall then have a mass really granular, and 

 which may, without any violation of probability, be supposed 

 to possess a regular external structure. A crystal thus com- 

 posed can have no regular cleavage, and would exhibit the 

 fracture, the imperfect transparency, and the optical structure 

 of Haytorite. 



Although this is a mere supposition with respect to Hayto- 

 rite, yet the smallest crystal of analcime is actually compos- 

 ed of twenty-four different solids ; and I possess crystals of 

 amethyst with perfect crystalline forms which are composed of 

 many hundreds of individual crystals, one half of which have 

 the direct, and the other the retrograde structure of plagiedral 

 quartz. Nay, in some crystals of this extraordinary mineral, 

 the combined individual crystals are so numerous that they re- 

 quire a microscope to be seen, and there are other crystals in 



