the Great Whin Sill, Northumberland. 247 



The knoh-iacets are characterized by a complex, crystalline 

 structure, sometimes appearing like striation, readily visible under 

 low powers (-nr ^^- objective), occasionally even with a hand-lens. 

 When well developed this structure seems to be that of a network 

 of felspar crystals, the ends of which, at times, make up the edge 

 of the facet, which has a serrated outline in conseqiunce. On a more 

 minute scale the appearance is that of striation, but this is resolved 

 under higher powers into a crystalline growth similar to the former 

 one, the apparent striation being produced by bundles of parallel, 

 close-fitting shafts. Some surfaces are structureless in parts, others 

 completely crystallized, and the orientation of the crystals varies 

 in different parts of the same facet ; a common feature, too, is the 

 intergrowth of two parallel bundles of crystals. The appearances, 

 in general, are very similar to those produced in the laboratory by 

 the crystallization of thin films of material on a flat surface, or 

 between two glass plates. 



Some specimens are traversed by wavy cracks from which the 

 crystal-shafts spring, or against which their tooth-shaped ends abut, 

 and when, as sometimes happens, these cracks are curved, they 

 usually enclose a crystallization distinct in direction from the 

 surrounding mass (PI. XVII, Fig. 2). 



The lines of apparent striation can often be resolved, under high 

 powers, into dark, aligned dots, which undoubtedly represent basic 

 matter eliminated in a lic^uid condition along the sides of the shafts 

 during their crystallization (PI. XVII, Fig. 6). 



When several facets meet with sharp boundaries they are 

 occasionally crossed by bands of crystalline growth, the orientation of 

 which is not altered in passing from one facet to another. 



The great complexity and beauty of many of these structural 

 peculiarities will be better realized by reference to the photographs 

 (Plate XVII). 



The facets of the mouldx resemble in detail those of the knobs, but, 

 in addition, they frequently exhibit dark, rectangular and rhomboidal 

 markings or impressions, only one example of which has been met 

 with on the knobs. Sometimes a single facet will show hnlf a dozen 

 such impi'essions, or again the line of intersection of two facets will 

 cut through one or more (even three) of them, each facet thus 

 claiming part of the impression. The edges of the impressions are 

 frayed at times somewhat in the manner of a skeleton-crystal, and, 

 in one case, the crystalline material enveloping the frayed edges 

 perfects the crude outline of the impression (PI. XVII, Figs. 1, 3). 



These appearances seem to imlicate that the impressions are in the 

 nature of negative crystals, produced by local dearth of crystalline 

 material, owing to the contact of the film with the surrounding 

 basalt. Where the contact is imperfect the small amount of 

 intervening crystallizable matter allows of the production of 

 a skeleton-crystal, and the combination of skeletal and Tiegative 

 crystal gives the frayed-edge effect. The comparative absence of 

 rhomboidal impressions on the knobs and their frequent occurrence 

 on the moulds is accounted for by the fact, established by the study 

 of the thin sections, that the knobs (the inclusions proper) consist 



