526 Corresjjondence—H. B. W.—Mr. TV. W. Waffs. 



and an annual rise of the sea of 3 inches of sea-level, a delta of 

 729 feet could have been formed by the deposits obtained by the 

 overflow of the river-water, with the assistance of some material 

 thrown back by the sea into the estuary or delta. 



The deltas of all our great rivers are thus later than Post-Pliocene, 

 and of the age of the Pluvial period. No part of any of these deltas 

 has been uplifted by volcanic or subterranean agency above the 

 general level of the delta ; this is another proof of recent origin. 



Alfked Tylor. 



EOCK-BASINS IN GEANITE. 

 SiK, — In reply to the query of Mr. T. Cragor in your last number, 

 I would refer him to a paper " On the Eock-Basins in the Granite of 

 the Dartmoor District, Devonshire," by G. W. Ormerod (Quart. 

 Journ, Geol. Soc. vol. xv. p. 16). In this paper the author brings 

 forward reasons for considering that the Kock-basins were formed 

 by atmospheric action, which commenced in irregularities on the 

 surface of the granite and was probably assisted by a globular or 

 spheroidal structure in the rock. H. B. W. 



JOINT-STEUCTUEE AT GEEAT DEPTHS. 



Sir,— Mr. Crosby (Geol. Mag. Sept. 1881, p. 416) explains the 

 absence of joint-structure at great depths by attributing the forma- 

 tion of these divisional planes to the cooling of strata from a 

 temperature which prevented them from becoming jointed by con- 

 traction before they were thoroughly desiccated and consolidated. 

 This appears to me to explain what occurs in jointed conglomerates, 

 in which hard quartz and other pebbles are often "cut through by 

 joints, as neatly as if they had been sliced by a lapidary's wheel." 

 But, if this is the cause of jointing, why have we joint planes 

 continuous in direction over wide areas, cutting rocks up into cuboidal 

 or polygonal masses, and not division along planes of least resist- 

 ance, such as would foi'm the prisms so familiar in rocks which have 

 cooled from fusion or from a high temperature like the columnar 

 mud of Tideswell dale. 



The conditions suggested by Mr. Crosby appear to me to be such 

 as would produce columnar jointing, viz. slow, regular contrac- 

 tion in a more or less homogeneous rock ; why then is not the joint- 

 ing of this nature ? Seeking purely for information on this head, 

 I am yours, &c., W. W. Watts. 



Sidney College, Cambridge, 

 October lUh, 1881. 



DISCOVEET OP COAL-MEASUEES UNDEE NEW EED SANDSTONE 

 AND ON SO-CALLED PEEMIAN EOCKS AT ST. HELEN'S, LANCA- 

 SHIEE. 



Sir, — Permit me to point out that the author of this paper in the 

 current number of the Geological Magazine, in identifying the 

 limestone bands met with beneath the New Red Sandstone at 

 "Winwick in 1879, with the Ardwick Limestones of the Manchester 

 Coal-field, does not state that this identification was made by me 



