GEOLOGY OF THE OUTER HEBRIDES. 565 



cohesion of the Conglomerate, the winds and waves have 

 played their fantasies in forming these sea-caves. The 

 entrance to the " Seal Cave " is evidently a washed-out 

 dyke. A fragment is seen forming the lintel to this sub- 

 terranean hall, the approach to which is along a narrow 

 and dark corridor, and must be made by a boat at all states 

 of the tide. 



TRAP ROCKS. 



The Island of Lewis is traversed from south to north 

 by huge trap or volcanic rocks passing to unknown depths 

 in the gneiss. The heads of these " dykes " sometimes 

 show in the Coast sections breadths of from twenty to thirty 

 feet, and where exposed to the sapping action of the waves 

 nothing save the lintel of the rock is seen over the door- 

 way of a sea cave. These trap dykes pass through the 

 conglomerate formation as well as through the underlying 

 gneiss. This shows that the conglomerates were laid down 

 before these " dykes " were formed. A good example of 

 these dykes and the parts they played in forming " geos " 

 and caves may be seen in the neighbourhood of Gress. 

 The "Seal Cave" there is entered through a door-way with 

 a fragment of the dyke forming the lintel. Owing to the 

 conglomerate having formed in huge sheets once this door 

 was forced the action of the waves in forcing and then 

 exhausting the air soon drew down sheets from the roof, 

 until within one finds himself in a spacious sea cave. The 

 pendents of carbonate of lime or " stalactites " from the 

 roof are formed by the water percolating through the 

 conglomerate. 



It is interesting to note that these Trap Dykes belong to 

 the volcanic rocks of the Secondary Age, and are probably 

 contemporaneous with the formation of those of the Car- 

 boniferous Age. The Shiant Isles, Cuchullin Hills of Skye, 

 the basalts of Mull, Staffa and the Giant's Causeway in 

 Ireland are all nearly in the same meridian. This line of 

 weakness on the Earth's crust runs on to the Faroe Isles 

 and Iceland. The Shiant Isles are, like the Bass Rock and 



