486 



JONES ON THE HAKTEELL-VALENTIAN [TSToV. IQOQ,. 



Fig. 7. — Vertical section 

 through a part of the 

 cyphus zone, shoiving 

 the 'pattern ' formed 

 by the flaggy beds. 



nodules occurs, -which can usually he seen near the southern hank 

 when the river is low. They are chosen as the upper limit of the 

 M.-rheidolensis zone, for the beds above these nodules are somewhat 

 shaly and are remarkably rich in graptolites, many of the species 

 of which have not been observed at a lower horizon. These beds 

 are only about 40 feet thick ; but, owing to the numerous un- 

 dulations and small faults which traverse them, they occupy the 

 bed of the river for about 400 yards across the strike. Their upper 

 limit is marked by another layer of large calcareous nodules, 

 exposed in the side of an old water-leat, made along the gorge 

 many years ago in order to supply some lead-mines lower down. 



Immediately beneath the lower nodules, in the crest of a small 

 anticline east of F. 13, Monograptas incommodus was obtained, 

 together with Orthograptus mutabilis and Climacograptus hvghesi. 

 For some distance the fossiliferous 

 beds underlying the upper nodules dip 

 towards the leat, and extensive bedding- 

 surfaces are laid bare ; some of these are- 

 covered with species of Monograptus 

 (chiefly M. atavus), which frequently 

 reach a length of 2 or 3 feet without 

 any appearance of a beginning or of an 

 end. 



A lithological peculiarity of these 

 strata may be mentioned here, as by its 

 means I was enabled to piece together 

 the detailed structure of this part of 

 the gorge despite numerous undula- 

 tions and faults, and to connect up the 

 sequence across a ' crush-belt ' which 

 obliterates for a space the stratigraphical 

 relationships. In the dark-blue shaly 

 beds there are sandy flags of a paler 

 colour, which vary in thickness from 

 mere stripes to about 3 inches. The 

 thicker flags occur at fairly-regular in- 

 tervals, and the spaces between them 

 are occupied by several of the thinner 

 flags forming a ' pattern ', which on a 

 waterworn surface is rather conspicuous 

 (see fig. 7). The character which at- 

 tracted my attention was the remarkable 

 persistence of these flaggy bands as the beds are traced along the 

 gorge, and after a little practice it became an easy matter to 

 identify certain conspicuous 'patterns' on opposite sides of a small 

 fault or crush-belt. I covdd detect no appreciable variation in the 

 thickness or spacing of the thicker flags in a distance of nearly a 

 quarter of a mile along the gorge. Having in this manner estab- 

 lished the stratigraphical order of the beds, I was able later to 



[Scale : 1 inch = 40 inches.] 



