44 Beports and Proceedings — Miner alogical Society. 



and its cliaracter is that of an intrusive junction rather than an 

 erosion surface. The fragraental deposits are believed to be of 

 volcanic origin and of the basalt age. The one exposed at the 

 seaward termination of the ridge is a volcanic agglomerate, probably 

 filling a small vent. The other, seen at the southerly base of the 

 Sgurr, is a bedded agglomerate, partly rearranged by water action. 

 The Torridonian and Oolitic sandstone blocks which are abundant 

 in it are held to have been brought up from below, and fossil wood 

 of Oolitic age has been brought up in the same manner. The 

 absence of fragments of the sill-dolerites (themselves younger than 

 the lavas, but cut off by the pitchstone) in both accumulations 

 seems to assign them unequivocally to the age of the basalt, and 

 tlieir conjunction with the pitchstone must then be considered 

 accidental. 



The conclusions arrived at bring the rock of the Sgurr of Eigg 

 into relation with the other British Tertiary pitchstones, which are 

 all intrusive. Thus also is avoided the difficulty of assuming a 

 great erosion in inter-volcanic times, a hypothesis for which the 

 .supposed river- valley was the sole evidence. 



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 position 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 last (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 in rock-slides 

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

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

 slide being freely rotated in a stage goniometer. With the higher 

 powers which are necessary for microscopic observations in 

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

 narrow limits. 



Dr. F. A. Bather exhibited fossils from various localities in New 

 2Iealand, hitherto known as " the Mount Torlesse Annelid," described 

 in the Geological Magazine (December, 1905, p. 532). Adds 

 further information thereon ; for which see his letter, p. 46 below. 



III. — Mineralogical Society of London. 

 November 14th, 1905; Professor H. A. Miers, F.R.S., President, 

 in the chair. — The Determination of the Angle between the 

 Optic Axes of a Crystal in Parallel Polarised Light, by Dr. 

 J. W. Evans. The crystal plate is rotated on the optic normal 

 as axis, and the positions are determined in which the relative 



