224 CONTRIBUTIONS TO MICRO-MINERALOGY. 



of Snow Crystals, throughout all of which Hexagonal 

 Symmetry prevails. I would draw especial attention 

 to fig. l4, a combination of obtuse Rhombohedrons, 

 with a central Hexagonal Prism : the relation of the 

 forms, and the symmetry of tlie grouping, is in strict 

 accordance with the Crystal-System to which Ice be- 

 longs, viz., the Hexagonal or Rhombohedric. These 

 symmetrical groups have been classed as twin-crystals, 

 a category to Avhich I fancy, on a broader view of the 

 subject, they will be found not to belong. 



3rd. Micro-Crystallogeny, comprising the influence of 

 foreign bodies, light, heat, electricity, magnetism, &c., 

 on crystallization as viewed in the field of tlie Micro- 

 scope. 



4th. Micro-Analysis, comprising methods of Structural, 

 Chemical, and Physical examination of inorganic 

 bodies, where the microscope is necessary as an aid to 

 the eye ; under this head will be given the method of 

 determining the Indices of Refraction in minute 

 prisms, *Scc. 



5th. Micro-Mineralogy. In this section will be given a 

 systematic Microscopical examination of mineral bodies, 

 from the simplest to those of the most complex 

 chemical composition, for the purpose of separating 

 from the true mineral Species, those that are of hetero- 

 geneous structure, and belong more strictly to the nature 

 of rocks. The following divisions suggests themselves : 



Division— HETEROGENIA. 



(^Oroups determined iy Micro- Analysis.) 



Of definite Chemical Composition. 



A. — Composed of two or more varieties of tlie same Species, or allied Spe- 

 cies associated in bands, concentric layers, &c., of which many 

 Agates are a type, 



B. — Homogeneous in aspect to the unaided ej'e, but composed in mass of 

 vegetable remains saturated with mineral matter, of which Coal is 

 a Tjq^e ; or conglomerated by mineral matter ; or consisting of the 

 inorganic hard parts of vegetables condensed into rock-like masses ; 

 this group might be termed Phytolites. 



C. — Phyto-Zoolites, an intermediate group between this and the next, 

 the organized constituents being a mixture of vegetables and animal 

 remains, of which Bergmehl is a Type. 



D. — Zoolites, the organized constituents being animal remains entirely, 

 of which Chalk is a Type. 



\^Part I. ivill appear in our next.] 



