CHAP. XXXIV 
PETROGRAPHY OF THE DYKES 
131 
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. 137). 
In connection with veins of finer material, that may belong to a late 
stage of the consolidation 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 
hy the still moving mass in the fissure. 1 
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 due to 
cooling and crystalline segregation in what was no doubt originally one 
tolerably uniform molten mass. The glassy central bands, too, though they 
mdicate a rupture of the dyke up the middle, may at the same time quite 
conceivably be, as I have said, extrusions from a lower portion of the dyke 
1 "-fore 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 a 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 
e ft after the separation from it of the crystallized 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 
"tirnr closely, can be shown to be the results of separate eruptions. These, 
' V uc k are obviously not exceptions to the general rule of the homogeneity 
0 dykes, I will consider in the next Chapter. 
Among the petrographical varieties observable in the field is the 
occasional envelopment of portions of the surrounding rocks in the body of 
' 1 } ke. Angular fragments torn off from the fissure-walls have been carried 
upwards in the ascending lava, and now appear more or less metamorphosed, 
( „ le am °unt of alteration seeming to depend chiefly upon the susceptibility 
the enclosed rock to change from the effects of heat. Cases of such 
1 See Mr. J. J. H. Teall, Quart. Journ. Geol. Soc. xl. (1884), p. 214. 
