248 THE TOADSTONES OF DERBYSHIRE. [Aug. 1907. 



chert. It seems more likely that we have here two sets of upper and 

 lower lavas, than that the cherty series has altered its horizon in a 

 distance of only 4 miles. 



The lava at Cressbrook Mill is covered by thin black limestones 

 with chert-bands ; it occurs therefore in the thin cherty-limestone 

 series, and is on a horizon higher than the Miller' s-Dale upper lava. 

 The limestones beneath it are seen in the road, and those above it 

 in a small quarry east of Cressbrook Mill. 



The Cave-Dale lava near Castleton is at least 200 feet below the 

 beds seen on Dirt Low, which are not the uppermost beds of the 

 Mountain Limestone. 



The Position of the two Main Lavas in the Series. 



The most reliable data for arriving at the horizon of the upper 

 lava-flow of the Millers-Dale area are to be obtained from the 

 section of Mountain Limestone seen along the Midland Eailway 

 between Monsal Dale and Buxton. 1 The section shows the upper 

 and lower lava-flows separated by about 150 feet of limestone, with 

 390 feet of more or less cherty limestones above the upper flow. I 

 have already stated that Mr. Sibly and myself consider that about 

 120 feet of massive limestone must be added to the Geological- 

 Survey estimate of beds above the upper lava. The following 

 section is thus arrived at : — 



Thickness in feet, 



: Thin limestones with chert about 400 



Thickly-bedded limestones 120 



Upper lava-flow 100 



Thickly-bedded limestones 150 



Lower lava-flow , 80 



Limestones — 



(See vertical section No. I, p. 249.) 



Therefore the upper lava of this district is about 520 feet down 

 in the series, and attains a thickness of 100 feet. The lower one is 

 seen at the bottom of Miller's Dale. The Geological-Survey Memoir 

 states that at the entrance to Chee-Tor Tunnel a third bed of toad- 

 stone breaks out : 



1 This will at first be of course taken to be the same as the bed already 

 mentioned as peeping up in the bottom of Miller's Dale.' (Op. cit. p. 20.) 



The officers of the Survey allude to the difficulty in making out 

 exactly how this bed of toadstone behaves, and consider that it is most 

 likely the same bed as that which crops out a little before reaching 

 the steep descent to Chee Tor. In order to prove whether there 

 were two or three lavas in the neighbourhood of Miller's Dale, 

 I made careful measurements of the thickness of the limestone- 

 beds which are exposed in the following dales : — Chee Dale, 

 Miller's Dale, Monk's Dale, and Sandy Dale, and also north of the 

 village of Chelmorton. The following table embodies the result, 



1 Geol. Surv. Mem. ' North Derbyshire ' 2nd ed. (1887) p. 18 & pi. ii. 



