64 JUiss M. K. Heslop i^ R. C. Burton— 



In the less disturbed places, where flow-structure is not greatly in 

 evidence, the fibres sometimes attain the size of ordinary skeleton 

 augites, which they resemble in general mode of occurrence. 

 Individual needles sink to the minutest dimensions, however, in 

 the disturbed parts and lose their parallel disposition, becoming 

 more felted together without definite plan or arrangement. These 

 differences may be taken as indications of the stages of inci])ient 

 crvstallizatiou produced under the two circumstances ; in neither 

 is "it far advanced, but those parts which solidified with moderate 

 quiescence allowed more o])portunity for the development of the 

 skeleton growths, whose decomposed form, supplemental by further 

 devitrification, we now see in the parallel bundles of green needles. 

 There is therefore no structural affinity between the base of the 

 tachylite and that of the adjacent ' stony rock'. 



Crystals are sparsely but uniformly distributed throughout the 

 slides, and are as abundant in the tachylite as in the ' stony rock'. 

 The porphjritic felspars seem to belong chiefly to the andesine variety, 

 but they approach labradorite in the central parts of the crystals and 

 pass into a more alkaline type in the outermost zones. Many of the 

 smaller laths must also be classed with andesine, and while some are 

 undoubtedly labradorite there is, on the other hand, a transition 

 towards oligoclase. On the whole, however, there is little range of 

 composition ; the majority belong to andesine, and exceptions in either 

 direction are rare. Those porphyritic crystals and groups which are 

 found in the tachylite are not much altered, but some of the porphyritic 

 individuals of the 'stony rock' are often greatly decomposed, faulted 

 by cooling cracks, and crushed, while others seem to have been over- 

 whelmed bv the 'lines of flow '. The smaller laths, binary twins for 

 the most ])art, are quite frequently bent or even broken, especially in 

 the tachylite. 



The large augites have suffered more severely from magmatic 

 cori'osion than the felspars. Some crystals are curved, some are 

 invaded by masses of glassy base, while all alike are characterized by 

 rounded angles. They seem to be tyi)ically rather broad octagonal 

 prisms of pale yellowish-brown colour, and are distinctly pleochroic. 

 The largest crystals were too much deformed to permit of a thorough 

 optical examination, but two smaller ones showed practically uniaxial 

 interference figures, which is in agreement with the character of the 

 older augites of the Cleveland Dyke. 



Small augites are by far the most abundant crystalline constituent 

 of the rock at this part. They occur in small beautifully sharp-edged 

 octagonal prisms, which are generally grouped together but rarely 

 intergrown. They are not noticeably pleochroic. There is none of 

 that bleaching of the groundmass in their neighbourhood, which is 

 frequently found in the vicinity of small ferro-magnesian minerals 

 embedded in a dark glassy matrix. The extinction angle on the 

 ^(010) face is from 35° to 40°. 



There are small groups of felspar often stellate in arrangement, 

 bound together b)' a central cluster of granular au<iite (Plate IV, AF in 

 Figs. 1 and 2). An apparent development of this on a larger scale 

 shows a central patch consisting entirely of granular augite — often 



