Tachylite of Cleveland Dyke. 63 



and its colour — the only distinction — is certainly the result of contact 

 with the highly ferruginous dark rock below it. A somewhat different 

 character was shown by a fresher specimen of the chilled margin 

 (PI. IV, Fig. 2) from which a section was cut, more or less parallel to 

 the contact surface of tachylite and 'stony rock'. The fresh glass in 

 this instance was brown, isotropic, and apparently quite amorphous 

 (BG, Fig. 2), for, though it was suggestively granular, no actual 

 grains could he focussed. Light-yellowish, bands (C, C) of varying 

 thickness (0-08-0'09 mm.) cross the dark glass in all directions, 

 dividing it up into somewhat angular areas, which, show faint but 

 distinct zones of colour (Z, Fig. 2), pale on the outside but grading 

 down to the normal dark brown at the centre of each patch. The 

 light-yellow bands (C, C) consist of distinct fibres (ch, Fig. 2), which 

 lie at right angles to a central crack. In every case a crack seems to 

 have been the starting-point for the devitrification process. There 

 can be little doubt that the cracks themselves are due to cooling. 



Lines of flow such as are seen in the layers beneath the tachylite or 

 even in transverse sections of the tachylite itself do not occur here, 

 but certain proof of flow-movement is available in the drawn out and 

 distorted bubbles, which are quite common (see b, b, Fig. 2). Some of' 

 them are black and opaque and seem to have an internal cavity, but 

 there are others (X) which present rather the appearance of a highly 

 refracting material of golden brown colour, not unlike rutile. They 

 are brilliantly polai'iziug, but clearly owe their interference colours to 

 small pleochroic laths felted together, which must either form the 

 Avails or be the decomposition product of some substance now filling 

 the gas cavities. There can be no doubt, from their parallelism and 

 general likeness, that the black-lined cavities and the yellow highly 

 polarizing areas are of similar origin, but that a deposit of iron-oxide 

 has produced the dark colour in one case, while devitrification is 

 responsible for tlie pleochroism and double refraction in the other. 

 The local concentration of iron-oxide here finds a parallel in the lower 

 layers, where alternating dark and light ' streams ' indicate the local 

 abundance and scarcity of granular iron-oxide. 



Porphyritic felspars and augites, besides small varieties of both these 

 minerals, occur plentifully in the tachjdite. All the crystals are 

 faulted by the cooling cracks when they lie in their paths, and, more 

 than once, small augites were seen apparently dividing the distorted 

 bubbles, thus showing that they were formed prior to the movement 

 which drew out the bubbles into their present elongated shape. 

 There is a perfectly sharp line of division between the tachylite and 

 the adjacent rock, and although practically the same crystals exist on 

 both sides of this line, the bases in which they are embedded are quite 

 distinct. 



The Stony Rock. — This consists mainly of masses of yellowish-green 

 fibres, among which lie small specks of iron-oxide ; these accumulate 

 in greater numbers and attain a much greater size in the dark area and 

 in the dark bands, whose meanderings indicate the flow-lines at this 

 part. It is to the iron-oxide specks that the granular appearance 

 of the ' stony rock ' is due, and an increase in their size and number is 

 apparently accountable for the darker regions of the base here. 



