NICHOLSON SKIDDAW GRArXOLlTKS. 



125 



above the bridge ; and as rain rarely falls, there is generally less 

 than a foot of water in the river-bed. This bed was nearly dry the 



Fig. 13. — Map of the distnct near Kurrachee. 





day after, as well as the day before this excessive rainfall. Other 

 instances of the same kind have been reported from India. The river 

 first banked up wood and grass against the bridge, and then threw 

 the girders over. The weight of these girders moved in a river-bed 

 of the dimensions given (Plate lY. fig. 11) * is an index of the rainfall 

 in Scinde, just as the fluviatile beds at Amiens are an index of the 

 current of the Somme, of its flood-level, and the tbrce of its stream. 

 "VVe cannot determine the rainfall at Amiens in the Quaternary 

 Period except by its results in the form of gravel - deposits ; and' 

 these appear to give as good indications as the fall of the Mulleer- 

 Bridge girders does of the flood in that river. 



2. The Graptolites of the Skiddaw Series. By Henry Alleyne 



Nicholson, D.Sc., M.B., F.G.S. &c. 



(Eead December 4, 1867 1.) 



[Plates V. & VI.] 



Contents. 



I. Introduction. 



II. Description of the Genera and Species of GraptoUtes. 

 III. Appendix, and List of the Species. 



I. Introduction. 



The Skiddaw Slates form the base of the great Silurian series of 

 the north of England, and comprise a group of rocks entirely without 

 admixture with interstratified igneous matter, attaining a thickness 

 of about 7000 feet, as calculated by Prof. Harkness. Lithologically 

 the series consists essentially of dark indurated shales, with a distinct 

 cleavage, " having intercalated through them coarser strata, almost 

 devoid of cleavage, possessing a flaggy nature, and affording fos- 

 sils " +. No distinct subdi\-isions can be drawn either on strati- 



* Figs. 10 and 11, Plate J V. are from drawings supphed by Mr. W. A. Brun- 

 ton, C.E. 



t For the otlier communications read at this Evening-meeting see p. 8. 

 J Quart. Journ. Gcol. Soc. vol. xix. p. 113. 



