400 A. Sfrahan — Explosive Slickensides. 



figured by Wright in his Monograph, a good figure of it in its 

 various stages would be an advantage.^ 



Besides these species, we have Am. elegans, Young, which belongs 

 to the genus Lioceras, and we have Am. bicarinatvs, Zieten, which is 

 distinct from any we have mentioned on account of its smaller centre 

 and furrows on the ventral area, and in all probability belongs to 

 another genus. I do not know if it has really occurred in England. 



It may be interesting to notice the sizes of umbilicus in these 

 various species taken from the figures given. The diameter = 100, 

 the umbilicus of Harp, falciferiim is, youth (Sowerby) 31-9 ; adult 

 (Wright) 40"5, showing the decrease of inclusion I have mentioned. 

 Harp, exaratum (Wright's figure) 20"5 and Harp, elegans (Sow. figure) 

 20'7. (These two have a different sectional view.) Harp, suhplana- 

 tum (D'Orb. pi. 114) 16-76, Am. hicarinatus (Zieten's figure) 12'2. 

 Whilst Wright's Am. bicarinatus, pi. 82, fig. 9, which I contend is 

 Sowerby 's Am. elegans, has, umbilicus 19 "00, which is almost exactly 

 the same as Sowerby's figure at the same diameter. I have given 

 these measures exactly from the figures. Of course in ordinary work 

 care would have to be taken as to the amount of body-chamber 

 present, and the size of the specimen ; but if allowance is made in 

 the present instance for these, it will not account for the variation 

 in the umbilicus in these species. 



F.S. — If Sowerby's Am. Strangeioaysi be really Eeinecke's Am. 

 serpentinus, I should much doubt, from the indications of suture- 

 lines given by Sowerby, if it be correct to class it in the genus 

 Hildoceras. Etymologically, as Dr. Haug wrote to me when making 

 his suggestion, Harpoceras is very aptly applied to the true Falciferi. 

 It is an important matter to assign a correct place to Waagen's genus 

 which had been partly forestalled by Hyatt's genera Tropidoceras, 

 Hildoceras, Lioceras, Grammoceras, Hammatoceras, and to another 

 section of which Bayle gave the name Ludioigia. In accordance 

 with these genera the restriction of Harpoceras becomes a necessity. 



lY. — On Explosive Slickensides. 



By AuBHEY Strahan, M.A., F.G.S. 



(Communicated by permission ol iLe Director-General of the Geological Survey.) 



DUEING a recent examination of the lead-mines of Derbyshire, I 

 was interested in some accounts of explosions which had taken 

 place, which were not due either to any material used by the work- 

 men or to fire-damp. Though at first inclined to believe that the 

 accounts were exaggerated, I soon found that not only was the 

 evidence of such explosions having constituted a real danger to the 

 men overwhelming, but that accidents are still liable to occur from 

 this cause. The explosions are connected with the structure known 

 as slickenside in the veins. The vein-stuff, consisting generally of 

 galena, calc-spar, heavy spar (sulphate of baryta), and fluor-spar, is 

 divided by the planes of slickenside into more or less vertical sheets 



1 Wherever Dr. "Wright has mentioned the zone of Harp, serpentinum, it must 

 probably be taken to indicate the zone of Am.falcifcr. 



