PEER ee NN SORT 
ie E 
1894.] Mineralogy. 599 
close approximation to a type as regard tal shape ( Krystallgestalt) 
and interfacialangles. Even when the symmetry of two ‘substances i is 
not identical, he makes comparison of the crystal shape as, e. g., between 
a cube and a rhombohedron with polar edge approaching 90°. The 
author distinguishes seven types as follows: I regular type (isometric), 
II magnesium type (hexagonal and pseudo-hexagonal—orthorhombic), 
III arsenic type (rhombohedral), IV quartz type (hexagonal tetarto- 
hedral) V @ tin type (tetragonal), VI rutile type (tetragonal and 
poeudo-tetragona]-orthorhombie), VII £ tin type (orthorhombic and 
pseudo ). Every group but the fourth con- 
tains metals and this type Rinne considers as derivable from the third 
or arsenic type. Many oxides, etc., have their crystal forms to some 
extent indicated in the forms of their contained metals. The term 
isotypism is proposed to describe these crystallographical relations 
between members of different divisions of the chemical mineral system. 
The author further states, “ It must now be accepted as a fact that such 
substances ” (elements, oxides, sulphides, haloid salts, and even silicates, 
which have been grouped together under his various types) “ possess 
equivalent or very similar crystal forms, and it follows that the chemical 
differentiation into elements, oxides, salts, ete., finds no crystallograph- 
ical expression, and therefore no independent, certain conclusion as to 
the chemical group to which a compound belongs can be drawn from 
its crystal form.” 
Lamellar Structure in Quartz Crystals.—In an “additional 
note on the lamellar structure of quartz crystals and the methods by 
which it is developed,” Professor Judd* describes and figures a remark- 
ably beautiful instance of lamellar structure in quartz, in which he 
sees a close analogy with the “rippled fracture” which he finds can be 
produced in quartz crystals by breaking them in a powerful vice along 
a plane perpendicular to the optic axis. The appearance of such 
fractures is very much like that of *engine-turned surfaces.” This 
appearance is caused by ridges following the planes R and -R, which 
are often curved and die out in the manner of plagioclase lamelle. 
From a study of the lamellie in an equatorial section of quartz sup- 
posed to be one of those investigated by Brewster, Professor Judd con- 
cludes that quartz is dimorphous. What' he calls “stable quartz” 
shows no tendency to assume a lamellar structure, whereas “ unstable 
quartz" constantly exhibits such a tendency. ‘The latter variety is 
usually amethystine. The lamelle consist of alternating bands of 
"Min. Mag., X, p. 123. 
