O’Rettty— Anatase and Brookite in Quartzites of Shankill. 739 
the density was found to be 3°928 (mean), which quite agrees with 
the mean value of the densities indicated for that mineral, viz. 
~—638'8 to 41. 
There is room for considering the probable origin of these 
titanic acid minerals as they occur in the Shankill quarries, and 
the clays therein found. It is to be borne in mind that minute 
erystals of rutile have been shown by microscopic examination to 
be present in most slate rocks, and, furthermore, that the presence 
of titanic acid has been proved by chemical analysis of various 
samples of the rocks of Bray Head, mostly as traces. It might 
therefore be assumed that the quartzites so characteristic of Bray 
Head and Carrickgollaghan would also show traces of that mineral, 
and its general diffusion through the whole of the strata of the 
*“‘Carrickgollaghan district,” in minute quantities only determin- 
able by very careful analysis. However, in the case of the 
Shankill quartzites there is evidence of thermal action in the 
abundant deposits of a ferro-manganese silicate along the main 
lines of fracture, as already mentioned, and in the cariated 
character of the quartzites in immediate contact with these ferro-. 
manganese silicates, as well as the altered texture of these quartz- 
ites, which are so friable and ‘‘ rotten” as to be useless for the 
purposes of road-metal. The examination of these particular 
quartzites in thin section, while showing the granular texture of 
the rock, points to a certain alteration in the structure of the 
quartz, such that it assumes the appearance of calcedony under 
polarised light, not to the point of showing the aggregation cross. 
on rotation, but an extinction seemingly due to the development 
of a fibrous structure which bears a certain relation to that of 
ealcedony. Another character of certain of the quartzites of Bray 
Head, points in this direction, that is, a relatively low density. 
Thus a specimen of quartzite from Windgate (Bray Head) 
analysed by Mr. Shegog (formerly Assistant Chemist in the 
Royal College of Science, Dublin), and marked by me No. 23, 
BH, gave a density, as determined by him, of 2°605, which 
corresponds well with that of calcedony, as given by Dana, viz. 
2°6-2°64, that of quartz being given by the same author as 2°653- 
2-654. It is right to add that the quartzite in question only 
contained 90°63 per cent. of Si0,, but the other constituents 
indicated in the analysis might, with reason, be looked as tending 
