High and Low- level Shelly Drifts around Dublin and Bray. 119 



embedded in situ on the shore, so that their axial direction 

 may be noted, when it appears that they are preponderatingly 

 in a N.W. direction. The stones are often well striated, much 

 more so than in the overlying drift, and the striae point in the 

 same north-westerly direction. The stones are not neces- 

 sarily horizontal, but lie at varying inclinations. Shells occur 

 in this clay. I took out a perfect Turritella and a fragment 

 of a very large Cardium, having a highly porcellaneous glaze 

 on the interior surface. Another curious feature is the pre- 

 sence of vertically-inclined beds of laminated clay, which 

 penetrate both the basement clay and the drift overlying it. 

 Vertical dykes of gravel also occur. 



A microscopic examination of the materials of this basement 

 clay showed that the larger grains were largely Cambrian 

 grit and schist, mica schist, and vein quartz, blapk chert, black 

 and grey limestone, quartzite, crystalline quartz, and a few 

 exceedingly rounded minute pebbles of black limestone. In 

 the smaller grains many well rounded and polished grains of 

 quartz occurred, and many flakes of mica. 



As illustrating this phase of the glacial deposits it will be 

 well to pay a visit to the Bog Hall brickworks about one mile 

 from Bray on the road to Kilmacanoge. The level of the 

 deposit is, roughly speaking, about 200 feet above the sea-level. 

 The clay of which the bricks are made is of a rusty brown 

 colour, and there appeared at first sight to be an unusual pro- 

 portion of Cambrian rocks among those which had been thrown 

 out. A further examination showed that there were some 

 large limestone boulders about, but these may have come from 

 the upper part of the clay. The clay seemed to be separable 

 into two beds. In the lower bed an excavation had been 

 recently made, and all the rocks thrown out appeared to be 

 Cambrian, Silurian, or granite. One block of Cambrian 

 measured 3 feet 6 inches long, and was well-planed and striated. 

 A block of quartzite conglomerate occurred, over 1 foot 8 

 inches long, and one granite boulder measured 3 feet in 

 diameter. A mechanical analysis of a specimen taken of this 

 clay told the same story, the larger residual fragments, after 

 washing and riddling, consisting of Cambrian rocks, schists, 

 grains of quartzite, quartz veins, &c. The smaller grains had 

 a larger proportion of quartz grains from the disintegration 

 of granite, but otherwise they were of the same character. 



What one would have hardly expected to find present in this 



A 2 



