TKANSACTIONS OF TIIK SECTIONS. 53 



SO, were they connected with outflows of dolerite, now wholly removed hy denu- 

 dation ? I confess that this supposition has often presented ilself to me as camdng 

 ■with it much proba1:)ility. It seems to me unlikely that so many thousands of 

 dykes should have risen so high as the present surface, retaining- there (as shown 

 by deep mines) much the same proportions as they show many fathoms down, 

 and yet that none of them should have reached the surface which existed at the 

 time of eruption. I regard it as much more probable that some of them, at least, 

 rose to daylight, and flowed out as coidees, even over parts of the south of Scot- 

 land and north of England, where all trace of such siu'face-masses has long been 

 removed. Some of the siuface-masses of dolerite in these districts may indeed be 

 of Tertiary age ; yet the proofs which the great Miocene basaltic plateaux present 

 of enoimous denudation are so striliing as to make the total disappearance of even 

 wide and deep lava-cuiTents quite conceivable. But a much more serious difficulty 

 remains. These dykes, as a ride, do not come up along lines of fault, j-et they 

 preserve wonderfully straight courses, even across fractured and irregular strata. 

 Each dyke retains, as a rule, a tolerably uniform breadth, and its sides are sharply 

 defined, as if a clean, sti'aight fissure had been widened .and filled up with solid 

 rock. In the coal-mines of Ayrshire, for instance, the miners have driven through 

 the dyke and found the coal, altered indeed, but at the same level, at the other 

 side. More than this, the dj'kes are found cutting across large faidts v^dthout 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 fiudts of lesser magnitude, 

 which follow roughly the dip of the rocks. But though these dillerent systems of 

 fissures already existed, and, as we might suppose, would have served as natural 

 pathway's for the escape of the subterranean 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 fractm-es produced? and how is it that they 

 shoidd run through all formations, up to and including the older parts of the 

 Miocene basalts, not as faidts, v\"ith a throw on one side, but as clean, straight 

 fissiu'es, 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 per- 

 haps additional evidence. 



Another featiu-e of oiu- 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 with a coarse agglomerate, consist- 

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

 strata. Such vents are sometimes not larger than a diniug-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 diff'erent inclined positions. The strata around them 

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

 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. Instructive sections of 

 these rocks abound along the coast line of Fife and East Lothian, and they occur 

 likewise in Ayrshire. One other part of the subject may be alluded to as deserving 

 of inquiry. There seem to be indications that local but well-marked metamor- 

 phism and the extravasation of syeuitic and granitic rocks have taken place in 

 connexion with some of our most recent volcanic phenomena. In Skye, Mull, and 

 Arran the association of such crystalline rocks with sheets or dykes of dolerite and 

 basalt shoidd be worked out carefull}\ The volcanic rocks of Britain are now 

 brought under the notice of the Section with the view of indicating a field of 

 research where much remains to be discovered, and [where the labourers are but 

 few. As a result of the neglect into which it has fallen, the nomenclature of this 

 portion of British geology has been virtually at a stand for about half a century. 



