Geological Society. 205 



points of inflexion lying in a line 



= - — 



7T 



= -5-7 

 parallel to the axis of x : this line is shown in fig. 2. 



(cl) The equation to a system of curves orthogonal to the 

 curves represented by (1) is 



x 2 +y i = s^ 

 where h' is an arbitrary constant. 



The direction of Ihe resultant force at any point, which 

 which would give rise to the curves represented by ( 1), is a 

 tangent to the orthogonal curve which passes through that 

 point. 



56 St. Swithin Street, Aberdeen. 



XXIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 180.] 



March 21st, 1900.— H. W. Monckton, Esq., F.R.S., Vice-President, 



in the Chair. 

 'T^HE following communications were read : — 

 - 1 - 1. ' On a Bird from the Stonesfield Slate. By Prof. H. G. Seeley, 

 F.R.S., F.L.S., V.P.G.S. 



2. ' The Lower Ludlow Formation and its Graptolite-Fauna.' 

 By Miss Ethel M. R. Wood. 



After dealing, with the literature of the stratigraphical and 

 palseontological sides of the subject, the author passes to a full 

 consideration of the sequence and character of the Ludlow Bocks 

 in the following localities : — The Ludlow district, the Builth district, 

 the Long Mountain ; and gives a briefer account of those of the Dee 

 Valley, the Lake district, Southern Scotland, Dudley, and the Abberley 

 Hills. While the Wenlock Shales are characterized by Gyrtograptus 

 and by the Flemingii-type of Monograptus, in the Lower Ludlow 

 Shales the colonus- and spinose forms of Monograptus such as M. 

 chimcera are abundant. The line between Lower and Upper Ludlow 

 is drawn at the top of the Aymestry Limestone. The Lower Ludlow 

 Bocks are divided into five graptolitic zones, which are not constant 

 in character or thickness in the different areas. The distribution of 

 the zones is given in the annexed table (p. 266). Two of the zone- 

 graptolites are new species, described in the latter part of the paper. 



In the Ludlow area the two lowest zones are rich in graptolites, 

 but shade into each other, and are therefore less clearly defined than 

 the higher zones, which each contain practically only one species 

 and are lithologically distinct. In the Builth area the variation in. 

 the zones in different parts may be due to the conditions of depth 

 and current under which the graptolites have been deposited, or to 

 the overlap of higher beds on lower, as has been shown to be the 

 case with the Wenlock and Old Red Sandstone rocks. Iu the Long 

 Mountain synclitie, M. scanicus is practically absent, and the 



