DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



43 



we arrive at a complete glass containing only the rudimentary forms of crystals 

 (globulites, longulites, &c), with scattered porphyrinic crystals of an earlier consolida- 

 tion. The process of the disappearance of this original glass may be admirably studied 

 in many dykes. At the outer wall, the glass remains nearly as it was when contact with 

 the cold walls of the fissure solidified it. From that external vitreous layer the 

 successive devitrification products and crystalline growths may be followed inwards 

 until in the central parts of the dyke little trace of the interstitial matter may 

 be left. 



The most instructive example of the process of devitrification which has come under 

 my observation occurs in the Eskdale dyke. The central " cores " already referred to 

 present a true glass, which in thin sections is perfectly 

 transparent and almost colourless, but by streaks and 

 curving lines of darker tint shows beautiful flow-structure. 

 The devitrification of this glass has been accomplished by 

 the development of crystallites and crystals, which increase 

 in number until all the vitreous part of the rock disappears. 

 What appears under a low power to be a structureless or 

 slightly dusty glass can be resolved with a higher objective 

 into an aggregate of minute globules or granules (globulites), 

 which average perhaps -^o.Vou °f an mcn m diameter. 

 Some of these bodies are elongated and even dichotomous 

 at the ends. These granules are especially crowded upon 

 clear yellow dart-shaped rods, which in turn are especially 

 prominent upon crystals and crystalline grains of augite 

 which bristle with them, while the immediately surround- 

 ing glass has become clear. There can be little doubt 

 that these rudimentary bodies are stages in the arrested 

 development of augite crystals. There occur also opaque 

 grains, rods, and trichites, which no doubt consist in whole 

 of magnetite (or other iron oxide), or are crusted over with that mineral. 



At least two broad types of microscopic structure may be recognised among the basic 

 dykes, (l) Holocrystalline, or with only a trifling proportion of interstitial matter. 

 This type includes the dolerites and basalts, as well as rocks which German 

 petrographers would class as diabases or diabase-porphyrites. The rocks are very 

 generally characterised by what is known as the ophitic structure, where the lath-shaped 

 felspars penetrate the augite, and are therefore of an earlier consolidation. In such cases 

 there is a general absence of any true interstitial matter. The rocks of this type are often 

 rich in olivine, and appear to be on the whole considerably more basic than those of the 

 second group. It is observable that they increase in numbers from the centre of Scotland 

 westwards, and throughout the region of the basalt-plateaux they form the prevailing 



* Proc. Roy. Phys. Soc. Edin., v. (1880) p. 255. 



Fig. 5. — Microscopic Structure of the 

 vitreous part of the Eskdale Dyke. 

 This section shows a crystal of augite, 

 enclosing magnetite and surrounded 

 with microliths, each of which consists 

 of a central pale yellow rod crusted 

 with pale yellow isotropic globulites. 

 The glass around this aggregation is 

 clear, but at a little distance globulites 

 (many of them elongated and dicho- 

 tomous) abound, with here and there 

 scattered microliths, some of which are 

 curved and spiral. (800 diameters. )* 



