O'Reilly — On the Calcite Crystals of Comity Antrim. 45 



forms, marked by ridges and furrows running nearly parallel to 

 the basal section, and giving rise to a rounded face. These oscil- 

 lating forms represent the scalenohedrons - 2i2" and - 5JR^. 



f Long edge, calculated, 153° 16', found, 153° 57' 



- 2i2^ I Short edge, ,, 



( Middle edge, ,, 



/ Long edge, 



- 5i?-t < Short edge, „ 



( Middle edge, ,, 



The roughness of the faces, and the rounded faces of the middle 

 edges rendered closer approximation difficult. The oscillation of 

 these forms gives to the surface of the crystal a feathered appear- 

 ance, the feathering proceeding out symmetrically on either side 

 from the red stripe representing the face - 211. 



In other vugs occurs a combination of forms somewhat different. 

 Thus, there is found a central pyramid of which the faces pre- 

 sent the roughness of a ground glass plate with cleavage planes 

 traversing them at regular intervals. This form represents the 

 pyramid -|i^2 : 



Calculated polar edge, 135° 51i', found, 135° 57'. 



On the alternate polar edges appear in process of formation, and 

 in different stages of that process, the faces of the scalenohedron 



Long edge, calculated, 161° 53', found, 161°. 

 Short edge, „ 101° 55', „ 101°. 



This last form is rarely complete, and its faces rarely cover the 

 whole of the underlying pyramid, and in the same manner as in 

 the case of the previously described crystals, leave a space or band 

 along the middles of the alternate pyramidical faces, but without 

 any red stripe. 



The conditions having led to the deposition of the red mineral 

 matter (peroxyde of iron apparently), present interest in so far as 

 they might lead to an estimate of the temperature of the solution 

 from which the crystals were formed. The only record existing as 

 to actual experiment in this respect is that of Senarmont (An. de 

 Ph. and Ch., t. xxx. p. 129), who transformed the hydrated 

 peroxyde of iron into anhydrous peroxyde by heating it in a 



