Geikie — Volcanic Rocks of Great Britain. 475 



found cutting across large faults without any deflection or alteration. 

 In short, no kind of geological structure, no change in the nature of 

 the rocks traversed, seems to make any difference in the dykes. 

 These run on in their straight and approximately parallel courses 

 over hill and valley for miles. The larger faults of this country 

 tend to take a north-easterly trend, and correspond in a general way 

 with the strike of the formations. At right angles, or more or less 

 obliquely to these, are numerous faults of lesser magnitude which 

 follow roughly the dip of the rocks. But though these different 

 systems of fissures already existed, and, as we might suppose, 

 would have served as natural pathways for the escape of the sub- 

 terranean melted rock towards the surface, the latter rose through a 

 new series of fractures, often running side by side with those of 

 older date. How were these new fractures produced, and how is it 

 that they should run through all formations, up to and including the 

 older parts of the Miocene basalts, not as faults, with a throw on 

 one side, but as clean straight fissures, with the strata at the same 

 level on each side? I do not pretend to answer these questions. 

 Let me only remark that had the trap-rock been itself the disrupting 

 agent it would have risen through the older fractures which already 

 existed as the planes of least resistance. The new fissures must be 

 assigned to some far more general force, of the action of which the 

 trap itself furnishes perhaps additional evidence. 



Another feature of our igneous rocks, deserving more special 

 consideration, is the occurrence among them of true vents, or the 

 sites of volcanic orifices. A very considerable number of these 

 vents is filled up, not with basalt, dolerite, or other melted rock (in 

 which cases the character of the mass as occupying an old vent is 

 apt to be less distiact), but with a coarse agglomerate consisting of 

 fragments of different trap-rocks, with pieces of the surrounding 

 sedimentary strata. Such vents are sometimes not larger than a 

 diniag table. In many cases, where the material filling them is fine 

 in texture, it is well stratified ; but its beds are on end, or thrown 

 into different inclined positions. The strata around them are much 

 indurated, and frequently, perhaps usually, are bent sharply down 

 round the margin of the vent, as if the ash or agglomerate, from 

 contraction or otherwise, had sunk and pulled the adhering strata 

 down with it. A careful mineralogical study of these vents, and of 

 the strata around them, would doubtless reward the observer with 

 the detection of many points of similarity to the products of modern 

 volcanos. Instructive sections of these rocks abound along the 

 coast line of Fife and East Lothian, and they occur likewise in 

 Ayrshire. 



It may be possible eventually to arrive at some approximate 

 realization of the form assumed by the surface of the country during 

 successive phases of volcanic action. There are, indeed, indications 

 that the eruptions were apt to occur along lines of broad valley. 

 The long depression, for instance, between the Highlands and 

 Southern Uplands of Scotland continued to be the site of active 

 volcanos during the Old Eed Sandstone and Carboniferous periods ; 



