68 Miss Heslop 8f R. C. Burton — Tachylite of Cleveland Dijke. 



been rammed up against the tachylite layer and even forced a little 

 way into it, breaking during the ])rocess. Evidence, too, of the 

 viscosity of the tachylite is found in one of its felspar laths which 

 curve with the curve of the junction line between the tachylite and 

 stony rock. 



The dark area probably cooled next and adhered to the firm surface 

 of the tachylite, while movement, either of flow or more probably of 

 contraction and contortion due to cooling aud settling down of the 

 great mass of the dyke, continued in the adjacent regions. For this 

 reason portions of the dark layer are torn down and tongues of paler 

 rock are forced right up to the tachylite. There can be little doubt 

 that the rapid formation of the film of tachylite retarded the cooling 

 of the adjacent magma, and thus the marked difference between the 

 glass and stony rock is accounted for, while their perfect contact is 

 due to the adherence of the continually cooling material to the only 

 firm surface in the vicinity, that of tlie tachylite, contact with which 

 further accelerated cooling. There would thus be a continually 

 increasing number of layers of viscous rock bound together more 

 closely than to the adjacent mobile material. Comparison with a 

 typical section of the Cleveland Dyke (from Cockfield) shows that the 

 porphyritic felspars have the same appearance and composition, but 

 while most of the smaller laths belong to andesine with a little 

 labradorite there is a noticeable development of more alkaline species 

 in the Cockfield specimen but not in the tachylite. This is readily 

 understood: the alkaline felspars are among the later crystallizations, 

 which were entirely checked in the tachylite. The large augites are 

 the same in both cases. The small augites of the typical rock are 

 about twice or three times as large as the corresponding crystals in 

 the tachylite, and while the former often have a surface coating of 

 black grains of iron-oxide, the latter are absolutely free from them. 

 Iron-oxide, indeed, does not occur as a primary crystalline constituent 

 in the tachylite. Comparison with other contacts, as well as with the 

 typical rock, shows that a marked suppression of the smaller felspars 

 must have taken place in the tachylite, while an impetus was 

 apparently given to the formation of small augites. This indicates 

 a local inversion of the usual order of crystallization, which generally 

 places felspar before augite in each generation in the Cleveland and 

 many other dykes. 



EXPLANATION OF PLATE IV. 



Fig. 1. Junction of Tachylite and ' Stony Eock ', Cleveland Dyke. 



la. Tachylite, pinkish-brown layer. 



lb. Bright-yellow (stained) band in tachylite. 



Ila. Dark layer of stony rock. 



L, L. Lines of flow, only shown here in tachylite, but very clear in the 



stony rock. 

 F. Felspars. F,, large, deformed, and partly decomposed individual. 

 A. Large augites. A] is a small prism, faulted and drawn up by a ' flow ' line. 



a. Small augites usually in groups. 

 ch. Areas occupied by green chlorite. Their shape is determined by the 



spherulites (only seen with crossed nicols) in the tachylite. 

 AF is a group of felspars with a central cluster of granular augite. 



