360 Mr. William Phillips on the OxydofTln. 



Fig. 26. shews the passage into the secondary pyramid, which 

 is complete in fig. 27. 



Fig. 28. represents an elongated crystal ; its elongation proceeds 

 from a regular deposition of crystalline laminse on one face of the 

 upper and on one of the lower pyramid, and on the intermediate 

 plane of the first modification. On the crystals represented by 

 fig. 29. a deposition of laminse has taken place on two opposed 

 planes of the first modification, gradually diminishing, so as to 

 preserve the lengthened faces of the second modification perfect 

 planes. On fig. 30. this species of deposition has taken place, after 

 the crystal itself had been formed similar to that of fig. 28. Fig. 31. 

 shews a crystal on which a regular deposition has taken place on 

 two opposed faces of the upper and the two corresponding faces of 

 the lower pyramid, so as to diminish two of the four triangular 

 planes of each, and to give the other two the form of irregular 

 hexahedral planes. 



Third Modification. 



This modification consists in a decrease on each apex of the 

 primitive form, by which each is replaced by a quadrangular plane, 

 perpendicular to the axis passing through the apices. Fig. 32. PI. 16. 

 The planes of this, though not uncommonly found in combination 

 with those of other modifications, are rarely so well defined as 

 to be depended on for accurate admeasurement, owing to an 

 unevenness on their surfaces. I have not succeeded in finding 

 crystals that have satisfactorily allowed the incidence of the planes 

 of this modification with those of the primitive form. 



Fourth Modijication. 



This modification consists in a decrease on each of the edges of 

 the primitive crystal, formed by the meeting of the two pyramids 



