Review of Dana's Mineralogy. 367 



same is described by Beck as occurring in the apatite of St. Lawrence 

 Co., N. Y. Six-sided prisms of calc spar are occasionally curved in the 

 same manner. 



" In many species the crystals appear as if they had been broken trans- 

 versely into many pieces, a slight displacement of which has given a 

 curved form to the prism. This is common in tourmaline and beryl. 

 The beryl from Monroe, Ct, often presents these interrupted curvatures 

 as represented in figure 7." 



In Vol. xlii, p. 206, of this Journal, Dr. John Locke described 

 some very curious instances of curved crystallizations of gypsum, 

 from Mammoth Cave, in Kentucky. Mr. Dana has given two 

 excellent figures of this species of distortion, drawn from speci- 

 mens in the collections of the National Institute at Washington. 

 Sir John Herschel has also described similar forms in ice on the 

 stalks of plants, (Phil. Mag. 1833, II, 110,) and we had the pleas- 

 ure of observing the same phenomenon recently on the stalks of 

 the Helianthemum corymbosum and H. Canadense. 



" Variations in the Angles of Crystals. — Variations in the angles ari- 

 sing from curvatures and imperfections of surface have been alluded to. 

 Other variations are owing to impurities in the crystal. Calcareous spar 

 is one of the most noted instances of this variation ; it varies from 105° 

 to 105° 17'. Pure crystals have the constant angle 105° 5'. These 

 variations are in general so small as seldom to cause any difficulty in 

 practice. Secondary planes, lustre, cleavage, and other peculiarities, 

 will always distinguish a cube from a square prism, although the angles 



differ but 1" from one another. 



♦ 



" From the investigations of Mitscherlich it is ascertained that the an- 

 gles of crystals vary with the temperature. In passing from 32° to 

 212° F., the angle of calc spar was diminished 8J-', thus approaching 

 the form of a cube as the temperature increased. Dolomite, in the 

 same range of temperature, diminished 4' 6". The angle of the prism 

 of arragonite was increased 2' 46" while passing from 63° to 212° F." 



3. Internal imperfections and impurities. — This is a head ca- 

 pable of much expansion. The controlling influence exerted 

 by the menstruum or medium, while minerals are taking on 

 their forms, particularly as regards the chemical constitution of 

 species, has not hitherto received sufficient attention. We have 

 no doubt that when this subject has been thoroughly inves- 

 tigated, much of the present complexity of the formulas given 

 for many species will vanish, and the small per cent, of many 

 matters discovered by chemical analysis and not essential to 



