48 Correspondence — Capt. Marshall Hall — Mr. S. Dyke Acland 



portions of those lavas that surround the inclusions and raising the 

 percentage of silica. He suggests that such a process at greater 

 depths and higher temperature may, under certain conditions, con- 

 vert a basic rock into a more acid one, so that possibly the andesite 

 of Strombolicchio may have been of basaltic character at an earlier 

 period of its progress towards the surface. He offers the suggestion 

 that other rocks or minerals once associated with the quartz have 

 been assimilated by the magma. 



coiRiRiBS iPOi srnDis^rcE- 



LEVEL OF LAKE LEMAN. 



Sir, — As Mr. Davison remarks, the level he quotes is not of 

 reliable precision — not adequately mounted. 



Would not the Pierre de Niton, in the Port of Geneva, be a more 

 satisfactory bench mark to watch and compare with selected stations ? 

 Upon its position turns the whole of the Swiss Federal Survey. 



Prof. Forel, who gives a full account of the results of levelling, 

 does not say much as to the systems, all of which depended upon 

 foreign [i.e. not Swiss) official surveys, so far as sea- level goes. 



On the other hand, as compared with stations on Swiss ground, 

 any variation of level ought to come out with certainty — and that is 

 the point. I hope this interesting matter will not be lost sight of. 



In Dufour's map the level of the Pierre de Niton figures as 

 376-64: metres. The later Siegfried map gives it as 376-86 metres, 

 a difference of 22 centimetres. Is this evidence of altered level or 

 of improved exactitude ? 



In times when the great Rhone ice-sheet was melting back from the 

 plains, but not as yet so notably from the mountains, would not 

 the disencumbered land westward rise more than at the east end 

 of Lake Leman ? See Osmond Fisher's ''Physics of the Earth's 

 Crust," second edition, page 327, note 2, for equations approximately 

 to the point. Marshall Hall. 



Easterton, Pakkstone, Doeset, 2nd Nov. 1893. 



VOLCANIC SEEIES IN THE MALVERN HILLS. 



Sir, — The excavations for the new reservoir to the east of and 

 below the Herefordshire Beacon have brought to light piecemeal an 

 interesting series of beds. At no one time was there a complete 

 exposure of them all ; but as I have watched the progress of the 

 work closely I have been able to make out with tolerable accuracy 

 their general bearing. 



The strike is nearly due north and south at this particular spot, 

 i.e. parallel to the axis of the Malvern Hills. The dip 40° East. 



The beds are shown by the microscoiDe to be (commencing from 

 the East) : obsidian or very fine ash, coarse ash, and some basic rock. 

 Some of the slides correspond in a remarkable degree with others 

 cut from rocks further south. I hope in time to be able to establish 

 connection between them. The whole area, however, has been 

 subjected to so much movement and shattering that the greatest 

 care and patience will be necessary to unravel the problem. 



GuEAT Malvern, Oct. ZQth, 1893. Henrt Dyee Acland. 



