Geological Society, 285 



Let tan (/> = -j , then 



0=40° 23' 10", 



and we must keep the middle point of the magnet on the line 

 making this angle with the meridian. 



XXXII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 145.] 



January 21, 1891.— A. Geikie, Esq., LL.D., F.R.S., 



President, in the Chair. 



HPHE following communication was read : — 



J- " On the Age, Formation, and Successive Drift Stages of the 

 Valley of the Darent ; with Remarks on the Palaeolithic Implements 

 of the District, and on the Origin of the Chalk Escarpment." By 

 Professor Joseph Prestwich, D.C.L., F.R.S., F.G.S., &c. 



i. General Character and Age of the Darent Valley. — The river 

 is formed by the union of two streams, the main one flowing east 

 from near Limpsfield, the other west from near Ightham, parallel 

 with the ranges of Lower Greensand and Chalk, and flows north- 

 ward into the Thames. The first indent of the valley was subsequent 

 to the deposition of the Lenham Sands, and indeed to the Red Clay 

 with flints and the old implement-bearing drift with which this is 

 associated ; and the same remark applies to a system of smaller 

 valleys starting near the crest of the escarpment and running into 

 the Thames. 



ii. The Chalk Plateau Drifts and associated Flint Implements. — 

 Since the publication of the author's Ightham paper, Mr. Harrison 

 and Mr. De B. Crawshay have found implements mostly of rude type 

 (though a few are as well finished as those of Abbeville) in numerous 

 localities on the plateau, where, owing to the gradients, the differ- 

 ence of level between plateau and valley-bottom is much greater 

 than at Currie Farm. Evidence derived from the character and 

 conditions of preservation of these implements is adduced in favour 

 of their great antiquity. 



iii. The Initial Stages of the Darent Valley. — The author has pre- 

 viously shown that in early Pliocene times a plain of marine denudation 

 extended over the present Yale of Holmesdale, and that in pre-glacial 

 times the plain was scored by streams flowing from the high central 

 Wealden ranges. These streams centred in the Darent, and the ex- 

 cavation of the present valley then commenced. There is a gap in the 



