72 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



to increase downwards ; that continued elevation might increase these fissures, but that 

 new fissures in the same direction would not arise in the separated blocks which would 

 now be more or less independent of each other; that subsequent subsidences would give 

 rise to transverse fissures, and by allowing the separated blocks to settle down would 

 cause irregularities in the width of the great parallel fissures. He considered also the 

 problem presented by those cases where the ruptures of the terrestrial crust have been 

 filled with igneous matter, and now appear as dykes. " The results above obtained," he 

 says, "will manifestly hold equally, whether we suppose the uplifted mass acted upon 

 immediately through the medium of an elastic vapour or by matter in a state of fusion in 

 immediate contact with its lower surface. In the latter case, however, this fused matter 

 will necessarily ascend into the fissures, and if maintained there till it cools and solidifies, 

 will present such phenomena as we now recognise in dykes and veins of trap." * 



The existence of a vast lake or reservoir of molten rock under the fissure-region of 

 Britain is demonstrated by the dykes. But, if we inquire further what terrestrial opera- 

 tion led to the uprise of so vast a body of lava towards the surface in older Tertiary time, 

 we find that as yet no satisfactory answer can be given. 



2. The rise of molten rock in thousands of fissures over so wide a region is to my 

 mind by far the most wonderful feature in the history of volcanic action in Britain. The 

 great plateaux of basalt, and the mountainous bosses of rock by which they have been 

 disrupted, are undoubtedly the most obvious memorials of Tertiary volcanism. But, 

 after all, they are merely fragments restricted to limited districts. The dykes, however, 

 reveal to us the extraordinary fact that, at a period so recent as older Tertiary time, 

 there lay underneath the area of Britain a reservoir or series of reservoirs of lava, the 

 united extent of which must have exceeded 40,000 square miles. 



That the material of the dykes rose in general directly from below, and was not 

 injected laterally along the open fissures, cannot be doubted. The narrowness of these 

 rents, and their enormous relative length, make it physically impossible that molten rock 

 could have moved along k them for more than a short distance. The homogeneous 

 character of the rock, the remarkable scarcity of any broken-up consolidated fragments 

 of it immersed in a matrix of different grain, the general uniformity of composition and 

 structure from one end of a long dyke to another, the spherical form of the amygdules, 

 the usual paucity of fragments from the fissure walls — all point to a quiet welling of the 

 lava upward. Over the whole of the region traversed by the dykes, from the hills of 

 Yorkshire and Lancashire to the remotest Hebrides, molten rock must have lain at a 

 depth, which, in one case, we know to have exceeded three miles, and which was probably 

 everywhere considerably greater than that limit. 



Forced upwards, partly perhaps by pressure due to. terrestrial contraction and partly 

 by the enormous expansive force of the gases and vapours absorbed within it, the lava 

 rose in the thousands of fissures that had been opened for it in the solid overlying crust. 

 That in most cases its ascent terminated short of the surface of the ground may reasonably 



* Op. cit., p. 69. 



