1909-10.] Tuesite — A Scotch Variety of Halloysite. 363 
Tuesite on the bank of a stream a few feet above the Tweed. It was on 
the slope below a place where Mr Thomson had obtained it when a boy. 
The river banks here are covered by trees and scrub, and there is no 
clear exposure showing the exact mode of occurrence of the Tuesite. But 
it is certainly not in the Red Sandstone, for it occurs in some igneous rocks, 
which, according to the Geological Survey map (Sheet 25), is a “ volcanic 
agglomerate in necks of Calciferous Sandstone age.” 
Tuesite does not occur as beds, and it was probably formed in veins by 
the action of hot ascending waters on the felspathic ash of the volcanic neck. 
Microscopic sections show that the Tuesite cannot be kaolinite, for it is 
amorphous and practically isotropic, and it is natural, therefore, to compare 
the material with halloysite, the amorphous hydrous silicate of alumina. 
The percentage of water, according to the two oft-quoted analyses by 
Thomson, is 14'2 and 13*5 per cent. ; and this amount would appear too 
low for halloysite unless the material had been dried at a temperature 
of above boiling-point. This proportion of water is that of kaolinite. 
Nevertheless, the material has been called halloysite by Dufrenoy * 
and Nicol,f though the chemical evidence on which they based their 
opinion would not alone justify this conclusion. The amorphous nature 
of the material as shown by the microscope, however, confirms their 
judgment. The fact that the material is found in small quantities, that it 
occurs as an alternation product in a volcanic neck, and not in beds in either 
the Old or New Red Sandstone, deprives it of the special interest it would 
have had if it had been a bed of kaolinite in a Palaeozoic or Triassic 
Sandstone. 
* A. Dufrenoy, Traite de Mine'ralogie, 2nd edit., vol. iii. , 1856, p. 585. 
t J. Nicol, Manual of Mineralogy , 1849, p. 222 ; Elements of Mineralogy , 1858, p. 170. 
Dana, on the other hand ( System of Mineralogy, 6th edit., p. 685), calls Tuesite a lithomarge, 
and it is included by Greg and Lettsom in kaolin or as closely allied to kaolin, lithomarge, 
and halloysite (Greg and Lettsom, Manual of Mineralogy of Great Britain and Ireland , 1858 
pp. 207, 448). 
(. Issued separately April 8, 1910.) 
