the Great Whin Sill, Northtimherland. 245 



thickness of tlie sill is only about 25 feet; the lower 15 feet is 

 massive, with a tendency to columnar structure, the rock free from 

 amygdules, coarse and breakable with eqnal ease in all directions. 

 Above this the rock is bedded, amygdaloidal, finer in grain, and 

 weatliers in lumpy form. The topmost bed, however, in contact with 

 the limestone, is free from amygdules, very fine in grain, vertically 

 jointed, weathers into sharp, jagged points, and splinters under the 

 hammer. 



Near tlie middle of the triangular patch, liigh up among the bedded 

 whin and just by the pool which is left at low tide, the rock takes 

 on a dyke- like aspect, is well jointed in directions east and west and 

 north and south, and breaks most readily along vertical planes. The 

 apparent dyke is about 6 feet wide and it runs east and west, that is, 

 parallel to the strike of the whin. In one place it forms an outstanding 

 feature several feet high, but it cannot be traced far, as it seems to 

 merge into the surrounding sheet. 



Tlie inclusions were first discovered in this dyke-like rock. The 

 whole exposure of whin yielded, on systematic examination, about 

 two hundred specimens, and the range of these was found to be 

 restricted to the dyke-like mass and the bed of trap with which it 

 appears to coalesce. The maximum dimensions of the rock with 

 inclusions are about 90 by 20 feet, and by far the greater number of 

 inclusions were found near the longer axis of tliis tract, which 

 coincides with the ' dyke' and its prolongation. 



The size of the inclusions varies from ^- to -| in., but is usually 

 about § in., the same as that of the ordinary amygdules. They are 

 characterized by a large number of facets, coated with a thin film of 

 a mineral having a bronze-yellow or, occasionally, a reddish colour. 

 When this film is chipped off a bright, reflecting plane surface is 

 revealed, and similar surfaces line the whin in which the inclusions 

 are embedded, the relations being similar to that of a mould and its 

 cast. These surfaces and films constitute planes of weakness, and 

 thus it happens that on breaking the rock a many-faced cast or knob 

 and its mould are simultuneou^^ly disclosed ; and, as the film clings 

 most tenaciously to the knob, this generally has a bronze colour, 

 while the mould is white and glassy. Occasionally the reverse is 

 the case, the film lining the mould; and, again, patches of the film 

 may be retained by both knob and mouhl, the bronze areas on the one 

 corresponding to the white higlily reflecting areas on the other. 



It happens comparatively rarely that a knob is broken across; in 

 such cases the inside usually appears to the naked eye to consist of 

 ordinary whin ; but at times, especially in weathered specimens, 

 a difference is discernible, tlie rock being darker, finer-grained, and 

 more easily scratched with a knife. When finely ground the powder 

 from the inclusions has a dark-grey colour, that of the whin being 

 somewhat brownish. On rare occasions weathered specimens show 

 a general spherical form, and it may be remarked, in anticipation, 

 that a corresponding circular outline is exhibited by the thin sections. 

 Only one hemisphere, however, shows the multitudinous reflecting 

 surfaces which are so characteristic of these inclusions. A completely 

 facetted inclusion has never been secured, either by accident or design, 



