60 Mks M. K. Henlop 1^ R. C. Burton- 



Ill. — Thk Tachylite of thk Cleveland Dyke. 

 By Miss M. K. Heslop, M.Sc, and E. C. Burton, B.Sc, F.G.S. 

 (PLATE IV.) 



rpHE Cleveland Dyke is well exposed low down on the left bank of 

 1 the River Tees, near the junction of this river and the Lune, 

 being washed at flood-times by the water. It trends in a direction 

 north of west, and can be traced for a distance of several hun^lred 

 yards: its thickness is difiicult to estimate as the north edge is 

 covered by drift. The tachylite variety of the rock is only exposed 

 for a few yards, and has only been found at this point — about 100 yards 

 west of the junction of the Lune and Tees. The dj^ke here appears 

 to occur in sheets very like successive lava-flows, and during cooling 

 a columnar structure has been developed and the bases of tlie columns, 

 mostly hexagons, face the river. The Cleveland Dyke at this exposure 

 is represented by three varieties of I'ock — 



1. Ordinary porphyritic rock. 



2. Amygdaloidal porphyritic rock. 



3. Stony rock associated with tachylite. 



It is with the third variety that this paper is concerned. The 

 stony rock occurs as a layer H feet thick, dipping 10° E.S.E. at 

 a point just opposite the junction of tlie two rivers, while a few feet 

 higher up the Tees the dip changes to 10° W.N.W. ; above and 

 underneath it the ordinary variety of the dyke occurs, and the 

 junctions are quite as sharp as between successive lava-flows. 



The true tachylite occurs as a selvedge \-\ inch thick, covering the 

 stony rock on its southern face for a distance of several yards, and 

 also seems to occur as veins in this variety. The latter mode of 

 occurrence is remarkable, and suggests that just before consolidation 

 part of the magma on the extreme edge of the dyke was injected into 

 cracks in the very viscous layer adjacent to it, the temperature of 

 which was low enough not to interfere with the almost immediate 

 consolidation of the injected material as tachylite. This explanation, 

 however, is put forward with a considerable amount of hesitation. 

 In some parts of the exposure the stony rock forms an inner selvedge 

 about 4 inches thick, while the tachylite covers this as a thin outer 

 selvedge. 



A few yards higher up the river there is an overflow of the dyke 

 to the south. A black shale is found lying horizontally underneath 

 the dyke for a distance of 1 1 yards ; we could not determine whether 

 the shale was greatly baked or not as the river water has softened the 

 rock and altered it ; the junction of the shale and the dyke is also 

 difficult of access. This fragmentarj^ section is, however, sufficient to 

 point to the existence of a small overflow to the south, and this 

 conclusion is supported by the occurrence of the igneous rock in sheet- 

 like form, where the tachylite is found; lower down tlie river the 

 ordinary dyke-form is resumed. The actual extent of the overflow, 

 as seen, is 1 1 yards, but it is probablj^ exposed over a greater distance. 

 Its existence is interesting as it seems to prove that, for some leasou 

 possibly connected with the hade of the dj'ke, the injection of the 

 molten magma has taken place with particular force to the south; 



