168 PROF. J. B. HARRISOX OX EXTRANEOUS [vol. lxxv, 



■obtainable from the naturally separated or weathered residua of 

 the limestone. With this object I examined, both chemically and 

 microscopical^, a sample of deep-red clay (which I had collected in 

 1889 from a newly opened pot-hole in the limestone of a road- 

 cutting at Castle Grant, at an elevation of about 1050 feet). 

 The results of the chemical analysis were as follows : — 



Per cent. 



Organic matter * 2*40 



Combined H.,0 7'60 



Chalcedonic Si0 2 7"30 



Organic opaline Si0 2 (sponge-spicules)... 9' 81 



Combined Si0 2 31'65 



A1. 2 3 23-98 



Fe 2 3 13-41 



FeO -32 



Ti0 o 2-33 



MnO -02 



CaO -08 



MgO -12 



K.,0 -36 



Na a O -61 



P 2 3 -015 



Total 100-005 



1 Containing N 2 *097. 



The high proportion of titanium oxide in the pot-hole clay 

 indicates the source of the minerals from which the clay was 

 derived to have been volcanic magmas, such as basalt or j^roxene- 

 andesite. My earlier examinations of the clay did not prove 

 quite satisfactory, as I did not succeed in breaking down the 

 chalcedonic silica contained in it, and hence obtained only scanty 

 indications of residual minerals. I renewed my examinations, and 

 found that the chalcedonic silica, which Avas resistant to solution 

 in a warm alkaline solution of a strength equal to 2 per cent, of 

 sodium hydrate, dissolved fairly readily in hot 20 per cent, solution 

 of sodium hydrate, although this liquid but partly attacked 

 and dissolved the more robust siliceous sponge-spicules present in 

 the residua. 



Microscopical examinations proved that the resistant minerals in 

 the residua consist of numerous minute grains of sanidine, the 

 larger with dulled, and more or less corroded surfaces and partly- 

 rounded outlines as if from the effects of incipient weathering ; 

 some of the felspar-fragments have inclusions of microlites or of 

 vacuoles, while one angular fragment shows the structure of 

 albite and some others that of microcline. In places, but rarely, 

 minute fragments of hypersthene, of a blue-green hornblende, and 

 •of tourmaline occur; while a few small fragments and some 

 splinters of brown and of green volcanic glass are present. The 

 edges of the ferromagnesian minerals are so worn that some 

 fragments are subangular. The dulled surfaces and partly 

 : smoothed-off outlines of the minerals in the pot-hole clay are those 



