Tachylite of Cleveland Dyke. 



61 



other overflows are known at Bolam and. Buckheads, near Bishop 

 Auckland, and have both taken place pre-eminently to the south. 



As seen in the hand-specimens the tachylite is a hard black rock 

 with a vitreous lustre, weathering to greyish on the surface. This 

 colour change due to weathering is also well shown under the 

 microscope, the tachylite becoming lighter near the devitrification 

 cracks. The glass was infusible in the Bunsen flames, but with the 

 blowpipe a black globule was formed, the fusibility lying between 

 two and three on the scale. The specific gravity from several 

 determinations is 2-60, while that of the ordinary variety of the 

 Cleveland Dyke lies between 2-63 and 2-788.' The hardness was 

 found to be 0'5. On treating some of the substance with hydro- 

 chloric acid to get rid of superficial iron-oxide, it was partly bleached 

 on the surface to a greyish colour ; this was thought to point to the 

 existence of free magnetite, but the tachylite proved to be non- 

 magnetic, being absolutely without effect on even the most sensitive 

 mirror-galvanometer. Some of the tachylite, when treated with acid, 

 seems to be heterogeneous, part being jet-black and part greyish- 

 yellow ; the latter probably due to a slight devitrification and 

 decomposition with formation of chlorite. 



The black variety was carefully sorted out and used for analysis. 

 The methods used were those described by Hillebrand in Bulletin 

 No. 422, Geological Survey, U.S.A. The results are recorded below, 

 together with the analyses of the ordinary variety of the dyke, of the 

 glassy groundmass as determined by Stock, and of the tachylite from 

 the Western Isles of Scotland.^ 



The composition of the tachylite is seen to correspond closely with 

 that of the ordinary variety of the dyke — the similarity is yet more 

 noticeable if other analyses of the dyke are considered. The particular 

 analysis given was chosen because its pei'centages of water and carbon 

 dioxide are the same as those in the tachylite. The silica percentage 



J. J. H. Teall, " On the North of England Dykes " : Q.J.G.S., vol. xl. 

 Judd & Cole, Q.J.G.S., vol. xxxix. 



