The Scottish Naturalist. 81 



time furnish excellent sections of the rocks which form the 

 island. 



The main geological features of Eigg are simple enough. The 

 great mass of the island is composed of sheets of basalt and 

 dolerite, piled one on the top of the other to the height of nearly 

 i ; ooo feet in some parts, while dykes of the same rocks cut 

 through these sheets in all directions. This volcanic mass is 

 underlain on the north and west sides by yellow sandstones with 

 shales, on the top of which is a thick stratum of clay. These 

 stratified rocks contain abundant organic remains. At the south 

 end of the island the basalts &c, are overlain by the rock of the 

 Scuir, a mass of columnar pitchstone-porphyry. Near the Scuir 

 dykes of pitchstone and obsidian intersect the basaltic rocks. An 

 acid lava is likewise met with near both ends of the island. 



In order to trace the geological history of this interesting island 

 it is necessary to go far back in the " record of the rocks," as far 

 back, indeed, as the middle of its great secondary division to the age 

 of the Middle Oolite, when we find that where Eigg now stands there 

 was an estuary or shallow-sea in which was being laid down white 

 or yellowish sands with gravel and boulders and other shallow-water 

 formations. The direction in which the land lay from which this 

 material was carried we do not know ; but, in all probability, it was 

 from the ancient mountains of Scotland, which then, as now, 

 marked the eastern limits of the sea ; at anyrate the shores of that 

 old sea were not distant, as the gravel and plant-remains included 

 in the sandstone amply testify. By-and-bye the conditions in some 

 way changed ; and fine clay was deposited on the top of the white 

 sands. (This Professor Judd has identified as the equivalent of 

 the Oxford Clay.) 



The fossils included in these sedimentary rocks show that 

 during these depositions the sea teemed with life, numerous 

 belemnites testify to the presence of extinct species of cuttle fishes 

 which must have literally swarmed in these waters; while ammonites 

 of various species, and Gryphia incurva and allied forms are found 

 in great profusion. Considerable quantities of the bones of reptiles 

 have been discovered, remains of some of the remarkable Jurassic 

 forms which have proved such a rich field of study to the natural- 

 ist and a kind of happy hunting-ground for the evolutionist. 



After the Oxford Clay was deposited, there comes a vast break 

 in the records of the history of the island. All through the Upper 

 Oolite, the Greensand and the Cretaceous periods, across the break 

 between the Secondary and Tertiary divisions of the rocks, and 



F 



