DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



41 



consists of a central more vitreous core and an outer more lithoid sheath. By the 

 coherence of their polygonal and irregular faces, and the greater durability of their 

 material, these sheaths project on the weathered wall of the vitreous centre of the dyke 

 in a curiously reticulated grouping of prominent ribs each about two inches broad (fig. 4), 

 while the vitreous cores, being more readily acted on by the weather, are hollowed out 

 into little cup-shaped depressions. Each rib is thus composed of the sheaths or outer 

 lithoid portions of two prisms, the line of separation being marked by a suture along the 

 centre (fig. 4, b). Between this median suture and the inner glassy core the rib is further 

 cut into small segments by a set of close joints, which are placed generally at right angles 

 to the course of the rib (fig. 4, c). Examined with a lens, the lithoid substance of these 

 sheaths has a dull finely granular aspect, like that of felsitic rocks, with scattered felspars. 

 It is obviously a more devitrified condition of the material which forms the core of each 

 prism. This material presents on a fresh fracture a deep iron-black colour, dull resinous 



a. b. c. 



Fig. 4. — Joint-Structures in the central vitreous portion of the Eskdale Dyke (B. N. Peach). 



a. View of a square yard of the outer wall of the vitreous central band, showing the polygonal arrangement of the prisms and 

 their investing sheath of ribs. 



b. View of a smaller portion of the same wall to show the detailed structure of the ribs (a, a) and their vitreous cores (b, b). 



c. Profile of a part of the weathered face of the wall, showing the way in which the hard ribs or sheaths project at the surface. 



lustre, and vitreous texture. It at once recalls the aspect of many pitchstones, and in the 

 early days of petrography was naturally mistaken for that rock. Through its substance 

 numerous kernels of more glassy lustre are dispersed, each of which usually contains one or 

 more amygdules of dull white chalcedony, but sometimes only an empty black cavity. 

 These black glistening kernels of glass, of all sizes up to that of a small bean, scattered 

 through the dull resinous matrix, form with the white amygdules the most prominent 

 feature in the cores ; but crystals of felspars may also be observed. Some details of the 

 microscopic characters of this remarkable structure will be given in a subsequent page. 

 The relation of the cores and sheaths to the prismatic jointing of the rock seems to show 

 that devitrification had not been completed when these joints were established, and that 

 it proceeded from the faces of each prism inwards. 



2. Internal Characters. — Much information has in recent years been obtained regarding 

 the microscopic structure of some of the basic dykes. The crystalline characters of those 



