240 MR. H. H. AENOLD-BEMEOSE OIT A SILL AND [May 1 899, 



band of vesicular and amygdaloidal toadstone from 1 to 2 feet 

 thick. This was succeeded by about 20 feet of large blocks of hard 

 dolerite or amorphous basalt with spheroidal structure in places. 



Both Mr. Wilson and Mr. Brown mention a thin bed of ash 

 below the clay ; the former writer considered that the clay was due 

 to an overflow of volcanic mud, and that it was coextensive with 

 the toadstone in this part of the valley. I have been unable, how- 

 ever, to find any trace of the ash-bed. According to Mr. Brown, the 

 clay-bed varied in thickness in diiferent parts of the quarry. This 

 is substantiated by a man who worked at the marble-quarry more 

 than 30 years ago, and who told me that the clay varied greatly 

 in thickness, and was entirely absent in some parts of the quarry. 

 Purther evidences of this variation in thickness are to be seen in 

 other parts of the Dale. 



It is important to notice that the greater portion of the toadstone 

 then seen was described as a hard, compact dolerite or basalt, with 

 only about 1 foot of vesicular rock or amygdaloidal rock below it. 

 The vesicular rock above the hard mass higher up the hillside does 

 not appear to have been examined. 



Mr. G. Fletcher described the toadstone in the quarry and also 

 some slabs of finely laminated limestone, the upper surfaces of 

 which were traversed by a network of fine cracks. He considered 

 that the cracks were sun-cracks in a coral-mud.^ 



Mr. J. Ward called attention to the spheroidal blocks of dolerite, 

 which he considered mainly owed their shape to a chemical change 

 in the rock, due to weathering.^ 



The first four observers attributed the induration of the clay to 

 a lava-flow, but do not appear to have noticed the great extent to 

 which the subjacent limestone has been marmorized. Though 

 Mr. Brown speaks of it as a coral-reef converted into a bed of hard, 

 massive limestone marble, it is uncertain whether he alluded to the 

 metamorphic effects of the igneous rock upon it, or whether he 

 merely used the word in the local sense for a hard limestone 

 employed for the same purpose as marble. He considered that, from 

 the varying depths to which the column descended in the clay, 

 important deductions might be made as to the persistency of the 

 heat of the superincumbent lava-bed at the time of deposit. 



Mr. H. B. Woodward speaks of it as a highly crystalline 

 limestone.^ 



In 1894 I had the privilege of visiting Tideswell Dale with 

 Sir A. Geikie, and pointing out to him the vesicular rock above 

 and below the more compact rock seen in the quarry. He then 

 discussed the probability of part of this toadstone being a sill, 

 and pointed out two faults, the southernmost of which cuts off this 

 toadstone and the subjacent limestones from the liniestones which 

 are higher in the series and contain two small lava-flows. In his 



1 ' Tideswell Dale Quarries,' Journ, Derby Arch, & Nat. Hist. Soc. vol. x 

 (1888) pp. 1-8. 



2 ' Further Notes on Tideswell Dale Quarry,' ibid. pp. 9-15. 



3 ' Geol. of Engl. & Wales,' 2nd ed. (1887) p. 159. 



