Prof. Maskelyne and Dr. Lang's Miner alogical Notes. 507 



very considerable, it is then difficult to see the differences of the 

 intensity of the light in turning round the crystal. In conse- 

 quence of these remarks, the prism-planes of the crystals of 

 Herschelite obtain the symbol 10 0; and the planes in the zone 

 [100, 001], 001 being the top plane, become planes of ho- 

 rizontal prisms parallel to the shorter diagonal of a prism 110, 

 on the planes of which the crystals are twinned. As none of the 

 planes are very good for measurement, the following crystallo- 

 graphic data of Herschelite can only be considered as approxi- 

 mations. 



The observed planes seem to correspond to the symbols 

 100, 301, 502, 201, 101, 001. 

 Taking for the elements 



a: b :c= 1:05774: 085 76, 

 we calculate for these planes the angles 



301.100 = 21 14 

 502.100 = 25 

 201.100 = 30 15 

 101.100 = 49 23 

 110.100 = 60 



The observed combinations are : — 



PI. VII. fig. 1.— 001, 5 2. 



On a crystal from Aci Reale, Sicily. One of these crystals gave 



5 2 . 5 2 = 49° 51' (50° 0' calc.) 



Fig. 2.— 100, 3 01, 101, 01. 



From Aci Castello near Aci Reale, Sicily. I found on a crystal, 



100.301 = 2l 56 21 14 calc. 

 100.101 =48 30 49 23 „ 

 101.101 = 98 46 98 46 „ 



These crystals, like the foregoing, occur on a sort of lava; 

 they arc often associated together io spherules, but are also found 

 single, imbedded in the vesicular mass, from which they can 

 easily be got out. Such crystals, very small but with tolerably 

 good planes, were combinations only of 3 1 and 001, with an 

 angle 



3 01.301 = 39|°-42° (42° 28' calc). 

 The top face of these crystals was also tolerably plane, but rough. 



Fig. 3.— 10 0, 2 01, 01. 



These crystals, from Cyclops, Catanea, are also interesting on 

 account of their association, coating with Phillipsite a mass of. 



2L2 



