226 Oil Crystallography . 



the line S a i: \. e. ihe secondary crystal would be a rhom- 

 boid tamed like the nucleus, and more obluse. ■ This case 

 is that of the binary oligiste iron mentioned above. 



Lastly, Can decrement take place in height ? In this 

 case the Faces produced, one of which is regarded as cor- 

 responding to the line K S m, will be rejected from the 

 other side of the axis ; whence it is easy to see that they 

 will incline towards the edges of the nucleus, in such a 

 manner that the secondary rhomboid will have a position 

 reversed with respect to that of this nucleus. 



Crvstallization presents some examples of these different 

 results. Those which relate to the two limits given by parallel 

 positions, or perpendicular to the axis, are constant, i. e. 

 they take place with respect to all possible primitive rhom- 

 boids. In the other cases the inclination of the faces pro- 

 duced by the same decrement varies according to the angles 

 of the primitive rhomboid. 



The striae and striges on the faces of secondary crystals, 

 when the operations of nature have not attained the per- 

 fection of which they are capable, frequently indicate, by 

 their directions, those which follow the edges of the laminae 

 of superposition ; and these accidents, which confirm the 

 theory in bodies capable of mechanical division, may also 

 show the progress of crystallization, and the direction of 

 the component laminae in those which are incapable of 

 mechanical division, and assist us in catching by analogy 

 the form and position of the nucleus, which otherwise 

 might escape observation. We should, hov.'ever, use with 

 caution the indications furnished by these accidents, since 

 it sometimes happens that the surface of the nucleus itself 

 is striated. This singularity seems to be the effect of an 

 imperfect decrement, which experiences such great inter- 

 ruptions, that the faces resulting from it sensibly coincide 

 with the primitive faces. In like manner, it is not im- 

 possible that the faces of a secondary crystal may have strisa 

 in a direction different from that which ought to result 

 from the progress of the decrements. But there are cases, 

 such as those of certain garnets with 24 trapezoids, in which 

 the strije are so palpable as to show plainly the mechauisia 

 of the structure, 



[To be continued.] 



XXXr. Pro- 



