IV PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. I906, 



Dr. J. W. Evans, in showing a new method of determining 

 the optic axial angle of a biaxial mineral, by rotating it 

 in parallel polarized light, on an axis at right angles to the optic 

 axial plane and to the axis of the microscope, said that the posi- 

 tions in which the relative retardation was zero corresponded to 

 the optic axes, and the angle between these positions was the 

 optic axial angle in air or in the medium in which the mineral was 

 immersed. To determine when the relative retardation is nil the 

 nicols are placed at angles of 45° with the axis of rotation ; the 

 double wedge described in the ' Mineralogical Magazine ' for May 

 1905 (vol. xiv, p. 29) is then inserted, and the position noted when 

 the bands on the two halves of the wedge are in exact continuation 

 of one another. 



This method is applicable to sections of minerals iti rock-slides 

 which are cut at right augles to the optic axial plane ; for the 

 observation can be made with a low power, which admits of the 

 slide being freely rotated iu a stage-goniometer. With the higher 

 powers which are necessary for microscopic observations in con- 

 vergent light, a rock-slide could only be rotated within very narrow 

 limits. 



Dr. F. A. Bather, in exhibiting fossils from various 

 localities in New Zealand, hitherto known as the 

 Mount-Torlesse Annelid, but described in the 'Geological 

 Magazine ' (December 1905) as Torlessia Machayi, a new genus 

 and species of tubicolous Polychasta, and as Dentaliam Huttoni, 

 sp. nov., explained that, relying on determinations of other fossils 

 by New Zealand geologists, he had supposed the age of the Mount- 

 Torlesse Shales to be not earlier than Carboniferous and not later 

 than Triassic. Prof. James Park, however, had shown that the 

 Maitai Series, with which these rocks were usually correlated, 

 was probably Lower Jurassic (Trans. N. Z. Inst. vol. xxxvi). 

 Since the volume containing Prof. Park's paper had not been 

 received either at the Natural History Museum or at the Science 

 Library, South Kensington, the speaker had been led into the 

 above error, which he desired to take the earliest opportunity of 

 correcting. 



In addition to the exhibits described above, the following 

 specimens and maps were exhibited : — 



Lantern-slides exhibited by Prof. E. Hull, LL.D., F.E.S., F.G.S., 

 in illustration of his paper. 



Specimens of rocks from the Sgiirr of Eigg, photographs (by 

 A. S. Reid, M.A., F.G.S.) and lantern-slides, exhibited by Alfred 

 Harker, M.A., F.R.S., F.G.S., in illustration of his paper. 



A weathered pebble from St. Bride's Bay (Pembrokeshire), 

 exhibited by J. V. Elsden, B.Sc, F.G.S. 



Geological Survey of England & Wales : 1-inch Map (Drift), n. s., 

 Sheet 283, Andover, by F. J. Bennett & C. E. Hawkins; and 



