90 PROCEEDINGS OF SOCIETIES. 



Analysis of Pitchstone from Lough Eshe, county of Donegal. 



Per-centage. Atoms. 



Silica, 64-04 1-423 



Alumina, 10-40 0-204 \ 



Eeroxide of iron, .... 9-36 0-104) 6Wm 



ime, 4-24 0-1511 



Magnesia, none . . . I n . q99 



Potash, 3-63 0-077 f ° 62Z 



Soda, 2-91 0-094J 



by ignition, . ... 5-13 0*570 



99-71 



Assuming this rock to be composed of quartz, felspar, and stilbite, 

 and writing Q, F, S for the number of atoms of each mineral respec- 

 tively, we find, since — 



Quartz = Si O 3 , 



Felspar - EO, Si O 3 + E 2 O 3 , 3Si O 8 , 

 Stilbite = EO, Si O 3 + E 2 O 3 , 3Si O 3 + 6HO, 

 Q + 4F + 4S= 1-423, 

 F+ S = 0-315, 

 6S = 0-570. 



From these equations we obtain readily, 



Q = 0-163, 

 F = 0-220, 

 S = 0-095. 



If we take 314 for the atomic weight of stilbite, which accords with 

 its usual composition, we find the following — 



Miner alogical Composition. 



Quartz, 7*33 



Felspar, 62-55 



Stilbite, 29-83 



99-71 



It would be very desirable that an investigation similar to the fore- 

 going were made into the composition of the different varieties of the 

 vast and heterogeneous family of trap rocks. The nomenclature of this 

 class of rocks is a reproach to geological science ; and no satisfactory 

 classification can ever be made of these rocks which is not based on their 

 chemical and mineralogical, as well as on their physical properties. 

 Why should not such a body as the British Association undertake the 

 task of reducing to order at least the British varieties of igneous rocks ? 

 The funds requisite for the investigation could easily be procured, and 

 the zeal of the members of the Association would supply specimens from 

 every locality of interest : and certainly an authorized nomenclature of 



