AND GEAVELS OF AKD'JTJN, ETC., IN MULL. 



273 



bles nothing so much as that of an electric discharge through the 

 air. It looks as if it had been injected with immense force while 

 molten, without reaching the surface ; and the sudden intrusion of 

 such a sheet may well have been accompanied by a violent earthquake. 

 Examples of these intrusive sheets are by no means uncommon in 

 the traps, and have been observed more particularly by Macculloch 

 and Geikie. They are readily distinguished by their compact tex- 

 ture and starch-like weathering amidst the piled up subaerial flows. 

 From the summit of the head the remains of apparently the same 

 sheets of perfectly horizontal traps can be seen stretching over 

 hundreds of square miles. It is w r ell known that these are believed 

 to be part of a Trap-formation that was once continuous from Antrim, 

 through the Inner Hebrides, to the Faroes, Iceland, and even Green- 

 land *. The included plants show that the flows were approximately 

 s3 T nchronous, speaking geologically, over the area, and they are emi- 

 nently representative of the type of massive eruption so graphically 

 described by Prof. Geikief ,who says : — " This association of thin nearly 

 level sheets of basalt, piled over each other to a depth of sometimes 

 3000 feet, with lava-filled fissures sometimes 200 miles distant from 

 them, presented difficulties which, in the light of modern volcanic 

 action, remained insoluble. The wonderfully persistent course and 

 horizontality of the basalts, with the absence or paucity of interstra- 

 tified tuffs, and the want of any satisfactory evidence of the thicken- 

 ing and uprise of the basalts towards what might be supposed to be 

 the vents of eruption, were problems which I again and again 

 attempted to solve. Nor, so long as the incubus of 1 cones and 

 craters ' lies upon one's mind, does the question admit of an answer." 

 The action of the Traps on the older sedimentary strata, shown by 

 Macculloch in his sections of the coast of Trotternish, in Skye, ap- 

 pears inconsistent with the view that they were poured out as lavas 

 from elevated cones. He illustrates a dyke which he speaks of as a 

 mile wide, giving off intrusive veins (' Western Isles,' vol. iii. pi. 17), 

 and which must have welled through long parallel fissures in immense 

 gushes, which appear to have flowed from seaward towards the 

 existing shore-lines, which in some cases still coincide with their 

 boundaries. 



Prestwich % advocates that the term Trap should be retained for 

 flows from fissure-eruptions, and Lava for those which have escaped 

 from craters. Whether, however, the present limits of the forma- 

 tion in this direction, even approximately, coincide with the original 

 ones is a question not easy to answer, in face of the colossal denuda- 

 tion to which they have been subjected. The traps at Burgh and 

 Carsaig are over 1000 feet in thickness, and are so horizontal that 

 they could scarcely have thinned so considerably within so short a 

 distance as Ardtun, where now no more than 150 feet remains. The 

 gneiss of Loch na Lathaich may, however, be part of an old ridge 

 against which they abutted. Their limits towards Benmore are far 



* Geikie, Quart. Journ. Geol. Soc. vol. xxvii. p. 279. 



t 'Nature,' November 4, 1880. 



\ Prestwich, ' Geology,' vol. i. p. 389. 



