350 Proceedings of the Royal Physical Society. 



written to Principal Dawson, Montreal, for information. 

 Principal Dawson, writing in the absence of Dr Hunt from 

 town, on E'ovember 5, 1879, says — " I write now merely to 

 state what I know as to the matter referred to. The 

 ferruginous coating which you mention is very common on 

 crystals of amethyst from Thunder Bay, and seems to have 

 been simply the latest coat of quartz deposited on the 

 crystals, and containing peroxide of iron in little rounded 

 hollow concretions with radiating spicules. This mode of 

 arrangement of oxide of iron is not unusual in reddish agates 

 from Nova Scotia and elsewhere, though with various 

 modifications in detail. I am not aware that it has been par- 

 ticularly described, nor that any special cause of it is known 

 further than the general one of molecular and crystalline 

 aggregation, which has to do duty in the explanation of an 

 infinity of curious forms in agates and other forms of quartz. 

 I cannot find that Dr Hunt has published any particular 

 account of the peculiar appearance in the Thunder Bay 

 amethysts." A thin slice prepared for the microscope, and 

 magnified ninety diameters, presented an appearance well 

 shown in the somewhat coarse, but highly characteristic 

 plate which accompanies this notice (see Plate XIX.). At 

 my request Professor Crum Brown kindly took charge of a 

 fragment which he entrusted to Dr Gibson for analysis, who 

 reported on the specimen as follows : 



" University of Edinburgh, March 21th, 1879. 

 " Report upon Crystal of Amethyst Quartz. 

 "A qualitative examination was made with a view to 

 determine the nature of the red colouring matter deposited 

 underneath the surface of the crystal. The result of this 

 examination showed the presence of iron, and the absence of 

 copper and other heavy metals. Ferric oxide being of very 

 common occurrence in quartz, there is no reason to doubt that 

 it is the red colouring matter in the crystal examined. 



"J. Gibson, Ph.D." 



My first impressions as to the colouring matter of the six- 

 sided crystals of this specimen were thus verified. It might, 



