DURING THE TERTIARY PEEIOD IN THE BRITISH ISLES. 39 



obviously been injected after the solidification of the rock which they traverse, they may 

 quite possibly be extrusions of a deeper unconsolidated portion of the same rock into 

 rents of the already stiffened overlying parts. The field-geologist cannot fail to be 

 struck with the much greater hardness of these fine-grained veins and strings that 

 ramify through the coarsely crystalline dolerite, andesite, or other variety of the broader 

 dykes. He can readily perceive in many cases their more siliceous composition, and the 

 inferences he deduces from the rough observations he can make in the field are confirmed 

 by the results of chemical analysis (see p. 44). 



In connection with veins of finer material, that may belong to a late stage of the con- 

 solidation of the general body of a dyke, reference may be made here to the occasional 

 occurrence of patches of an exceedingly compact or homogeneous texture immersed in 

 the usual finely crystalline marginal material. They look like angular and subangular 

 portions of the more rapidly cooled outer edge, which have been broken off and carried 

 upward by the still moving mass in the fissure.* 



In general, each dyke is composed of one kind of rock, and retains its chemical and 

 mineralogical characters with singular persistence. The difference of texture between 

 the fine-grained chilled margin, with its occasional glassy coating, and the more coarsely 

 crystalline centre is obviously due to the effects of different rates of cooling in what was 

 no doubt originally one uniform molten mass. The glassy central bands, too, though 

 they seem to indicate a rupture of the dyke up the middle, may at the same time quite 

 conceivably be, as I have jsaid, extrusions from a lower portion of the dyke before the final 

 solidification of the whole. The ramifying veins of finer grain that now and then traverse 

 one of the large dykes are likewise explicable as parts of the same stage towards entire 

 consolidation. All these vitreous portions, whether still remaining as glass or having 

 undergone devitrification, are more acid than the surrounding crystalline parts of the rock. 

 They represent the siliceous " mother-liquor," so to speak, which was left after the 

 separation from it of the crystallised minerals, and which, perhaps entangled here and 

 there in vesicles of the slowly cooling and consolidating rock, was ready to be forced up 

 into cracks of the overlying mass during any renewal of terrestrial disturbance. 



But examples occur where a dyke, instead of consisting of one rock, is made up of two 

 or more bands of rock which, even if they resemble each other closely, can be shown to be 

 the results of separate eruptions. These, which are obviously not exceptions to the 

 general rule of the homogeneity of dykes, I will consider in a later section of this paper. 



Among the petrographical varieties observable in the field is the occasional 

 envelopment of portions of the surrounding rocks in the body of a dyke. Angular 

 fragments torn off from the fissure-walls have been carried upwards in the ascending lava, 

 and now T appear more or less metamorphosed, the amount of alteration seeming to depend 

 chiefly upon the susceptibility of the enclosed rock to change from the effects of heat. 

 Cases of such entanglement of foreign substances, however, are of less common occurrence 

 than might have been expected. Occasionally, where the enclosed fragments are oblong, 



* See J. J. H. Teall, Quart. Jour. Geol. Soc, xl. (1884) p. 214. 



