120 



DR' GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



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sheets may be counted among the shales and limestones. They are sometimes not six 



inches thick, and seldom exceed six or eight feet. # 



It is observable generally throughout the west of Scotland, that the rock of the sills 



is more coarsely crystalline than that of the dykes, which in turn is generally not so fine- 

 grained as that of the bedded basalts, but that the 

 development of the crystalline structure is usually 

 proportionate to the thickness of the mass. On the 

 whole, the thinnest sheets are finest, and the thickest 

 are coarsest in grain. But as they never reach the 

 dimensions of the Antrim mass, they do not present 

 such a largely crystalline texture as can be seen at 

 Fair Head. It is further noticeable that, while they 

 tend to lie between the bedding planes of the rocks 

 which they traverse, they frequently break across 

 them. What is most singular in this respect is their 

 occasional disruption of one of the solid columnar 

 or amorphous basalts, as in the example from Eigg 



^\ just cited, where one might have supposed that the 



ig. »— ec ion o siow e e an path of least resistance would have been much more 



Intrusive Sheets, Eigg. r 



readily found along the line of junction between two 

 beds. Again, the abundance of intrusive sheets about the base of the volcanic plateaux 

 contrasts strongly with their scarcity or absence higher up. We may examine miles of 

 the central and higher parts of the basalt-escarpment without detecting a single example 

 of them, but if the escarpment is cut down to the base we seldom need to search far to 

 find them in numbers. 



If we consider the facts which have now been adduced regarding the position and 

 structure of the Intrusive Sheets, we are led, I think, to regard these masses as belonging 

 to the history of the basalt-plateaux, but to a comparatively late part of it. They 

 consist of essentially the same materials as the lavas that form these plateaux, though 

 with the difTerences of structure that the conditions of their production would lead us 

 naturally to expect. Where they occur in thick masses, they have obviously been able to 

 cool much more slowly at some depth beneath the surface than the comparatively thin 

 beds could do that were poured out above ground, and hence they have there assumed a 

 far more largely crystalline texture than was possible for them under other conditions. 

 Their extraordinary number about the base of the basalt-escarpment points, in my 

 opinion, to the increasing difficulty which the gradual thickening of the basalt-series 

 presented to the uprise of molten matter. That the plateaux were rent open and that 

 lava rose in the fissures thus caused, even after a depth of 2000 or 3000 feet of basalt had 

 been piled up, is proved by the height to which dykes can be traced in Mull and else- 

 where. But there would no doubt come a time when the vents would grow fewer, and 



* Quart. Jour. Geol. Soc, xxvii. (1871) p. 297. 



