522 Mr. W. 8. Gresley — Explosive SUckensides. 



abundant than in the strata above (Calj)) and below (lower limestone 

 with shale partings). I would also draw his attention to the papers 

 published by the Boston Society on the island of Cuba, which I 

 suspect might throw some light on the subject. As suggested in 

 previous writings I sus]3ect that the cherty zones in the Irish 

 Carboniferous Limestone, especially that between the Fenestella 

 Limestone and the Calp, must have some connection with vulcanicity. 



Years ago Jukes got chert from Queen's Co., Limerick, Clare, etc., 

 examined by Sorby, and I think 1 remember that he published about 

 them. 



Geological Surtey of Ireland. Q-. H. KiNAHAN. 



EE "EXPLOSIVE SLICKENSIDES." i 



Sir, — I should like, if I may, to add a few facts which seem to 

 closely bear upon the subject of Mr. A. Strahan's interesting article 

 in the August Number of your Magazine. They are these : — In 

 driving, exploring, or "opening-out" headings in certain seams of 

 coal, loud reports are very frequently heard, which are often accom- 

 panied by the bursting-off from the sides of the excavations of large 

 blocks or masses of coal. The noise made hj such "explosions" or 

 reports may be likened to artillery, and often causes men to run out 

 of the place with alarm. Now, these " bumps," as the miners term 

 them, generally occur in situations where the strata are much faulted 

 by dislocations, and increase in importance with depth or thickness 

 of cover. They probably happen most frequently and loudest in 

 single drifts or headings, or those formed in advance of the general 

 workings of the mine ; and it is where these excavations are formed 

 in the lower part of the coal-seam that the " bumps " are heaviest 

 and produce greatest effects. Such an instance occurred a few years 

 ago in one of the pits of the Moira Collieries close here, when a 

 sudden and very severe bump completely displaced and shattered. 

 a single-brick " brattice "-wall (or partition, dividing the excavation 

 longitudinally for ventilating purposes) for a length of about 24 feet. 

 This wall was, as it were, completely Mown out, and the men in the 

 place were "jumped-up " off the floor, but not hurt. The wall was 

 about three feet high and built with mortar. Again, in excavatimg 

 the main roadways in the solid coal in the thicker seams of South 

 Staffordshire, very severe bumps take place, and have been known 

 to suddenly displace hundreds of tons of coal, by throwing them off 

 the sides into the road. But in the ordinary course of coal-getting, 

 especially by the method called the " Longwall " {i.e., where all 

 the seam is extracted by one operation), loud reports with bumps 

 are of every-day occurrence, and now and again they have the effect 

 of knocking out the props and sprags (wooden supports to roof and 

 sides) and bring down a quantity of stuff. Also, during the 

 operation of "holing" (under-cutting the coal-seam preparatory to 

 breaking it down), the coal will keep on bursting itself off in little 

 fragments from the face of the excavation with loud explosive reports, 

 otten putting the men's candles out. When the coal does this, it is 

 said to have plenty of " life " in it, or " it keeps talking to you." 

 1 See Prof. T. McKenny Hughes' article, ante, pp. 5U-512. 



