MEXICAN TYPE IN THE TOPAZ. 27 



In the Brazilian type of Topaz (figure 1), the crystals are characterized by the pre- 

 dominance of the tri-polar form or pyramid P. This is always accompanied by the 

 vertical prism V, or by Y and V2 ; both, as a rule, highly lustrous, but strongly striated 

 longitudinally — -the striae indicating other vertical forms. The base is almost invariably 

 absent. The colour of these crystals is deep-yellow or wine-yellow, changing commonly 

 to pink or red on ignition. 



The Siberian type (figure 2), to which also the Aberdeen and Japanese crystals be- 

 long, is characterized by the predominance of the side-polar or brachydome 2P ; with 

 vertical planes as in the preceding type, but, as a rule, much less coarsely striated. The 

 basal plane is frequently absent, or, if present, is narrow or small in size ; and the planes 

 of other forms, as P, etc., are also essentially of small dimensions. The colour is typically 

 some clear shade of green, deepening into bluish-green ; but the crystals of this type 

 from Japan are practically colourless, and their polar planes (2P, |P, P — the two latter 

 quite subordinate) are commonly dull or granulated, as though coated with a siliceous 

 etfloresceuce. 



In the Saxon type (figure 3) the characteristic form is the base, B, always well or 

 largely developed, but mostly without lustre. This is generally bordered by several 

 polar forms (2P, 4P, P, 2P, etc.,) but all of small size and frequently of little more than 

 linear dimensions. The vertical forms comprise V and V2, and occasionally V3, with 

 finely striated planes. The form V2 predominates in some cases to the exclusion, or 

 nearly so, of V, giving the crystal a marked rectangular aspect, as the angles of this 

 prism equal respectively 86° 49' and 93° 11'. The same peculiarity is occasionally shown, 

 however, by crystals of other types, notably by many of the Aberdeen crystals. Examples 

 of this Saxon type generally present a pale tinge of yellow, but some are practically 

 colourless. 



In crystals of the Mexican type (figure 4), to which attention has not to the writer's 

 knowlege been hitherto directed, the characteristic form is the comparatively rare side- 

 polar or brachydome 4P, accompanied by the pyramid 2P — both largely developed, so as 

 to impart a strikingly acute aspect to the crystals. The pyramid P is also frequently, and 

 the base B occasionally, present, but these forms are hardly characteristic. In most, but 

 not in all examples, the polar planes are more or less dull ; but those of the vertical 

 prisms Y and Y2 are comparatively bright, and sufficiently free from striie to give good 

 reflecting surfaces for measurement. These Mexican crystals are chiefly pale yellow, with 

 a slight brownish tinge, and thus, as regards colour, they fall between the Brazilian and 

 Saxon varieties. 



a flattened or obtuse one, with its macro- or right and left axis four times the length of the vertical axis. Proper- 

 ly, the symbol should he written 014 instead of 041 — the brauhy-axis being always assumed in this notation to be 

 placed first, the macro-axis in the centre, and the vertical axis third. Another objection to this raethoJ of notation, 

 is, that it is only adapted to the eye, not to tlie ear. When employed verbally, it can hardly be followed without 

 a certain amount of confusion or mental strain, even when used by a comparatively slow speaker. It has also the 

 defect of making the notation of a plane refer more to the axis cut by the plane than to that to whicli the plane 

 is parallel : whereas it is evident that, from a physical point of view, forms are in closer relationship to the axes to 

 which tliey are parallel, than to those which they cut, as long recognized in the employment of the terms bracby- 

 pinakoid, brachydome, macro-pinakoid, etc. 



