90 HEMITBOPE AND INTERSECTED CRYSTALS. 



that they are so produced. The arrangement of the 

 molecules in so apparently capricious a manner, is 

 doubtless the consequence of some law operating on 

 the structure of the crystal from the commencement 

 of its formation, and analogous to those laws by 

 which other secondary forms are produced. 



Hemitrope crystals consist of portions of either 

 unmodified primary forms, or of secondary forms; 

 and the plane of the imaginary section is found to be 

 parallel either to the primary planes, or to some 

 secondary plane which would result from some regu- 

 lar decrement. 



Oxide of tin, as shewn in Mr. Phillips's paper on 

 that substance in the 2nd vol. of the Geological 

 Transactions, exhibits a considerable variety of this 

 species of secondary form. It is very common also 

 in felspar, in rutile, and sphene, and it occurs in 

 many other substances. 



One character by which hemitrope crystals may 

 generally be known, is the re-entering angle produced 

 by the meeting of some of their planes. This is very 

 obvious in the figures 114 and 116. But even where 

 this re-entering angle does not appear, there is gene- 

 rally some line, or some other character, which in- 

 dicates the nature of the crystal. 



Crystals are frequently found intersecting each other 

 with greater regularity than can be ascribed to acci- 

 dent, and forming a class very analagous to hemi- 

 tropes, and probably governed in their structure by 

 the same general laws to which those forms owe their 

 existence. 



