130 Scientific Proceedings, Royal Dublin Society. 
forms are those of sodium chloride; he has also made drawings 
for me of one of the sections from Ballymote (see Fig. 1). 
Fig. 1. 
Chert Slice. Ballymote, Co. Sligo. 
Magnifying power used, " (scale approximate). 
{n order to observe the forms in most instances a rather high 
power is necessary. I finda one-inch objective with No. 2 eye- 
piece, magnifying 100 diameters, generally the most serviceable. 
With this power the crystals are distinct, but in cases where the 
crystals are very minute, the quarter-inch objective and No. 2 
eye-piece, magnifying 350 diameters, is necessary. 
The general appearance of the crystals is variable. Where the 
siliceous paste of the chert is clear and colourless, the crys- 
tals generally occur also colourless and slightly glistening, the 
edges and angles alone of the planes being apparent as fine, sharp 
slightly darkened lines or points, as the transmitted light is in- 
terrupted. On the other hand, where the enclosing siliceous paste 
is darkened, the crystals are also often darkened along the edges 
and angles. In many cases it seems to me probable that the 
actual sodium chloride crystal is absent, has, in fact, disappeared, 
its cavity only being left, and that the walls of the cavity are 
darkened by iron oxide, or by carbonaceous dust. The presence 
of sodium in minute quantities is indicated in some of the speci- 
mens analyzed by Mr. Hardman, F.C.S., the analyses of which 
are given in our joint paper on the “ Nature and Origin of Chert.”* 
* Hull and Hardman ‘‘ On the Nature and Origin of Beds of Chert, &e. ” Scientific 
Transactions, Royal Dublin Society, Vol. I. (New Series), Part VII. (1877). 
