382 Geological Society. 



was found that while the loss in twenty-four hours in the gold spring 

 was 8™ 4% that of steel G"" 25% and that of palladium 2"" 31% that of 

 a glass spring was only 40^. These differences the authors ascribe 

 principally to the different degrees in which the substances had their 

 elasticity reduced by an increase of temperature. As glass was thus 

 found to suffer a much smaller loss of elasticity by this cause than 

 metals, they proceeded to construct a glass balance suited to the cor- 

 rection of the small error still occasioned by this cause, employing a 

 glass disc for this purpose. The compensation being completed, they 

 next tested the isochronism of the glass spring, and it proved to be as 

 perfect as any metallic spring. Chronometers thus constructed are 

 now in course of trial at the Royal Observatory. In common with all 

 other instruments of the same kind they have shown a disposition to 

 progressive acceleration, the cause of which is but little known, but 

 which appears to be influenced by the action of the air. 



GEOLOGICAL SOCIETY. 

 (Continued from vol. viii. p. 380.) 



April 13. — A memoir on the Geology of Coalbrook Dale, by Joseph 

 Prestwich, Jun., Esq., vvas commenced. 



April 27- — The memoir on the Geology of Coalbrook Dale, by Joseph 

 Prestwich, Jun., Esq., began on the 13th of April, was concluded. 



In a paper read before the Society in February 1834, Mr. Prest- 

 wich gave an account of some of the principal faults of this coal-field, 

 and in the present memoir describes fully the extent and physical 

 features of the district, the formations of which it consists, the dis- 

 locations not previously noticed, the superficial detritus or drift, 

 the organic remains, and the inferences which the author conceives 

 may be drawn from the facts enumerated. In the first place, how- 

 ever, he acknowledges the assistance which he has received from Mr. 

 Murchison, Mr. Anstice of Madeley, and the gentlemen connected with 

 the coal-works ; he also acknowledges the aid which he has derived 

 from Mr. Arthur Aikin's labours in the same district. 



The coal-field is bounded on the east by a slightly undulating line 

 ranging from Lilleshall to Bridgenorth ; on the north-west by a line 

 nearly coincident with the main road from Lilleshall to Watling-street, 

 near Wellington, and thence by the Wrekin; on the west the boun- 

 dary is broken by the gorge of the Severn, but is formed, in part, by 

 the elevated ridges of Benthall and Wenlock ; and on the south-east 

 it is defined by the road from Much Wenlock to Bridgenorth. 



The area thus circumscribed consists of a platform raised about 400 

 feet above the Severn at Madeley, or 500 above the level of the sea ; 

 the surrounding country seldom rising to a height exceeding 350 feet. 

 It is intersected by numerous picturesque glens, including the cele- 

 brated defile through which the Severn flows at the Iron Bridge, and 

 is traversed by several low hills, the most elevated of which is about 

 746 feet above the level of the sea ; but the Wrekin, which forms part 

 of the north-western boundary, rises to the height of 1320 feet. 



The formations of which the district consists are, commencing with 

 the oldest, 1st, some members of the Lower Silurian rocks; 2ndly, 

 the Wenlock and Ludlow rocks, belonging to the Upper Silurian sy- 



