62 MisH M. K. Heslop S^ R. C. Burton— 



in the glassy rock is slightly lower — further analyses would probably 

 o-ive a slightly higher figure ; the sum of the lime and magnesia is 

 somewhat lower, while the sum of the oxides of iron, aluminium, and 

 titanium is higher than in the ordinary variety of the dykes, and the 

 alkalies are practically identical. 



These results, then, agree with the evidence as to the identity of 

 composition of the tachylite and ordinary variety of a dyke obtained 

 by Judd and Cole from a study of the dykes of the Western Isles of 

 Scotland, which we will now briefly refer to. 



We know of but one more example of a glass}' rock of this kind 

 from the North of England dykes,' so that it will be interesting to 

 compare the Teesdale tachylite with that from the Western Isles of 

 Scotland. The dykes differ fundamentally in the presence of olivine 

 in the Scottish rocks and its absence from the Cleveland Dyke — at all 

 events, at this exposure the latter is an augite-andesite, while the 

 former are described as olivine basalts ; the non-magnetic character of 

 the Teesdale tachylite also distinguishes it, and it seems probable that 

 the oxide of iron exists in a different form in this rock, as ferric 

 oxide, or at least not combined to form magnetite as seems to be 

 the case in the Scottish variety. The latter is also an exception to the 

 rule that the specific gravity of the tachylite is less than that of the 

 ordinary variety of a dyke ; we have already seen that the tachylite 

 of the Cleveland Dyke conforms to this rule. It is interesting to 

 note that both the glassy rocks contain corroded augites and felspars, 

 showing that the conditions of intrusion of the two magmas must 

 have been similar. In chemical composition there is a great 

 resemblance between the two rocks. 



When examined under the microscope the slides show a glassy rock 

 passing abruptly into a larger mass of true tachylite about i inch in 

 thickness at the outer margin; the sections fall naturally into the 

 following divisions (see PL IV, Figs. 1, 2) : — 



y rr. 1 1-^ ( (<?) Pale pinkish-brown layer. 



i. iachyiite | J^^ Bright yellow layer in contact with {a). 



I (rt) Dark granular region of irregular limits merging 



ir St • k ) ^^^'^ 



tony loc < ^^^ Light-grey area in which swirling black streams 



\ still mark the ' lines of flow '. 



Flow-structure, indeed, is evident in both the main divisions of the 

 section : in the tachylite the lines run parallel to the junction ; in the 

 stony layers beneath they curve and twist about much more, at 

 a little distance below the actual contact, but with a general trend in 

 its direction. 



The Tachylite (PlatelV). — The spherulitic nature of the tachylite is 

 quite plain in ordinary light, for the presence of chlorite in the interstices 

 serves greatly to emphasize the outlines of the spherules. The yellow 

 l)and. Fig. 1 (lb) differs in no important point from the upper tachylite, 



' A slide of this rock in the collection of Dr. Teall was examined, with his 

 kind permission, by Miss Heslop; it is marked " Tachylite, 85 miles east of 

 Greenhaugh ", and contains well-formed porphyritic augites (with large optic 

 angle) not unlike those of the Cleveland Dyke, but they are pierced by ground- 

 mass felspars. 



