238 Reports and Proceedings — Mineralogical Society. 



Boulder-clay near Hatfield and at Finchley. The section of the river 

 between liickmansworth and the Eastern Drift had its direction of 

 drainage reversed, and the water escaped at Rickmans worth by a new 

 channel which became the Colue. 



On the melting of the ice, Fluvio-glacial Gravels (Plateau Gravels 

 of some writers) were left over a great area. These gravels are 

 composed chiefly of Eocene and Cretaceous materials derived from 

 the Gravelly Diift, but also contain Bunter pebbles which have been 

 brought down the Thames. The Bunter pebbles are particularly 

 abundant in a band between Bourne End and Watford ; to the south 

 only a few, presumably washed out of the band, are found. Thej- 

 are believed to show that the Thames in times of flood returned to its 

 former course. 



The floods from the melting ice, added to the water's of the Thames 

 and Colne, produced, \yj denudation of the Eocene clays, the great 

 flat through which the Thames now flows, east of Maidenhead, and 

 which, opposite Iver, is 8 miles wide. 



After the retreat of the ice, the Wye and Misbourne extended their 

 channels over the Fluvio-glacial Gravel flat, and some other small 

 streams were formed. 



2. " Some New Lower Carboniferous Gasteropoda." Ey Mrs. Jane 

 Longstaff {yiee Donald), F. L.S. (Communicated bv Dr. G. B. Long- 

 staff, M.A., F.G.S.) 



Eight species of Gasteropoda are described, six being regarded as 

 belonging to five new genera or sub-genera, the others representing 

 Pithodea, De Koninck, which has not previously been recorded from 

 the British or Irish Carboniferous Limestone. Among others, the 

 shell of Pleiirotomaria [Tropidostropha) Grtffithi, M'Coy, is described 

 in detail, and the nature of the fine pitting of its extei'nal surface is 

 discussed. 



II. — Mineralogical Society. 



March 12, 1912.— Professor W. J. Lewis, F.R.S., President, in 



the Chair. 



Dr. G. F. Herbert Smith and F. JS". A. Fleischmann : On the 

 Zeolites from Killyflugh and White Head, County Antrim. Chabazite 

 occurs in three different kinds of crystals and gmelinite in two, and 

 the former is also found pseudomorphous after calcite. Analcite 

 occurs in clear trapezohedra, and natrolite in fine needles. Tlie 

 character of the occurrences was described. — Dr. J. Drugman : On 

 Quartz Twins. Further specimens of bipyramids twinned on the 

 primary rhombohedron from the Esterel, France, were shown, thus 

 establishing this mode of twinning, which was first described by. 

 Q. Sella in 1858. From the same localities were shown also 

 bipyramids twinned on ^ (1122), in which, too, the prism is absent 

 and there is no flattening perpendicular to the twinning plane as in 

 the Dauphine and Japanese specimens. — T. V. Barker : Note on the 

 Optical Properties of Mercuric Iodide. Preliminary determinations by 

 means of two 30° prisms gave 2-746 and 2'447 as the values of the 

 ordinary and extraordinary refractive indices sodium, and 2-566 and 



