Correspondence — Mr. A. B. Wynne. 18§ 



COiaiilESIPOn^lDIEIsrOIE]. 



WHAT IS AN ERRATIC? 



Sir, — I am constrained to ask the above question in consequence 

 of the appearance, in the Records of the Geological Survey of India 

 (vol. X. p. 223), of some critical remarks by Mr. Theobald, upon my 

 previous reference to the " Erratics " of the Upper Punjab in the 

 same volume (p. 123). 



In these remarks Mr. Theobald restricts and applies the term 

 " Erratics " exclusively to certain blocks supposed to have been 

 ice-transported, advocating the idea also {vide foot-note) that the 

 word is only applicable in describing recent phases of geology. 



Every field geologist must have observed travelled rock-fragments, 

 the connexion of which with glaciation was not apparent, and to 

 which the comprehensive term " Erratic " would be perfectly ap- 

 plicable. Such transported fragments might have come from some 

 neighbouring mountain, or have been transported by a large river, 

 or drifted by a marine cixrrent, or ice might have moved them. In 

 all these cases I hold that the fragments would be truly erratics, no 

 matter at what period of geological history they wandered ; also 

 that glacial erratics may be included in the general term. 



Chalk flints are found in the shore deposits of the South of Ire- 

 land. They may have come from France, or the South of England, 

 from Antrim, or from unknown Chalk beds beneath the English 

 Channel, but there is no evidence that they have been transported 

 by ice. The materials of the Chesil Bank are accumulated by 

 marine currents. The agenc}'^ which enclosed Wicklow granite masses 

 far within the Carboniferous Limestone of the County Dublin ;. or 

 the great blocks of granite in a flow of Deccan Trap near Mund- 

 laisir, on the Nerbudda, in India, are unknown : but are not these 

 as genuinely erratics as any ice-borne blocks ? If not erratics, what 

 else are these travelled fragments to be called ? 



The ei'ratics of the Upper Punjab are of various kinds. One 

 group of them dates very far back ; these are all fragments of 

 crystalline rocks, including many of red granite from an unknown 

 source, and they occur imbedded in the geological series of the Salt 

 Eange, at every stage from pre- Silurian to latest Tertiary or Eecent. 

 Another group is of Himalayan origin; these have been transported 

 from the north during Tertiary, post-Tertiary, and Eecent periods, 

 and are still travelling from the same direction : they include a great 

 variety of Himalayan rocks. Yet another group comprises large 

 angular and sub-angular blocks of Himalayan gneiss, granite, lime- 

 stone, etc., which are supposed to have been carried by floating-ice. 

 Nothing has yet been advanced to connect them with glaciation of 

 the immediate localities in which they are found. 



Some very large angular, red granite erratics, at considerable 

 elevations on the Salt Eange, and resting on diff'erent strata, are 

 probably also attributable to flotation by means of ice. The best 

 known of these is the Khewra Erratic. 



