Notices of Memoirs — Dr. T. G. Bonnet/ — Origin of the Trias. 519 



III. — Exhibition of a Kemarkable Form of Sodalite from 

 Kajpdtana. By T. H. Holland, F.R.S. 



"VTEARLY every discovery in the interesting family of nepheline 

 X\ syenites sbow^s some feature of unusual interest amongst 

 igneous rocks. The latest discovered occurrence of these rocks in 

 India is remarkable for the presence of a form of sodalite which 

 has the property, apparently unique amongst minerals, of rapidly 

 changing colour in bright daylight from carmine to pale grey or 

 colourless, and of slowly recovering its carmine colour when kept in 

 the dark. The mineral with these peculiar properties was discovered 

 by Mr. E. Vredenburg as a constituent of the pegmatitic veins in 

 a nepheline syenite intruded into the Aravalli schist series of 

 Kishengarh in Rajputana. Along the same belt the sodalite, 

 intergrown with nepheline in the pegmatite veins, is of the common 

 blue variety, and nothing unusual is shown by chemical analysis of 

 either variety. The carmine colour disappears as rapidly on exposure 

 to light in a moist atmosphere as in dry air, in the cold weather as 

 rapidly as at higher temperatures, and under bright electric light as 

 in daylight. The mineral has apparently no effect on a photographic 

 plate, and is not noticeably radio-active. The reappearance of the 

 carmine tint takes place in a few weeks in some specimens, but 

 requires some months' concealment in the dark in others. No 

 explanation has been offered so far to account for this remarkable 

 phenomenon, and the specimens are now exhibited with the hope of 

 obtaining suggestions for a systematic investigation of the mineral. 



IV. — On the Origin of the Trias. By Professor T. G. Bonnet, 

 ScD., LL.D., F.R.S. 



rilHE three subdivisions of the Bunter, whether east or west of the 

 .J_ Pennine Range, apparently unite to the south of it, and thin 

 out as they approach the southern parts of Warwickshire, Stafford- 

 shire, and Leicestershire. Their equivalents are fairly well developed 

 in Devonshire, but apparently thin out in a similar wedge-like 

 manner towards the north and nortli-east, not reaching the Bristol 

 Channel. The upper and lower members in the northern area are 

 sandstones, generally red, often conspicuously current-bedded, but 

 without pebbles, the grains being frequently wind-worn. The 

 pebble-bed reaches a thickness of 1,000 feet near Liverpool — where, 

 however, sand dominates over pebbles — is about 300 feet thick in 

 the northern part of Staffordshire, and rather overlaps the Lower 

 Bunter sand. The writer describes the lithological characters of the 

 pebbles, and discusses the reasons for and against deriving them 

 either from a southern or south-western source, like those in the 

 Devon area, or from any region, either exposed or buried, in their 

 more immediate neighbourhood, maintaining a northern origin to be 

 the more probable. The Keuper group, both sandstones and marls, 

 extended without interruption (except for the sea) from Devonshire 

 to Yorkshire on the one hand, and Antrim on the other. 



