THE LOCH LOMOND AND OTHER VALLEYS. 215 



portions of what is now the lake-bed were above the 

 level of the sea, underground vents may possibly have 

 existed, as we know they do in other parts of the world. 

 Suppose the country about Loch Lomond to be 600 

 feet higher than at present, and rain and rivers to 

 be excavating the basin. This hole being at the 

 junction of the main and Culness valley breaks, it 

 is possible that in this place there might have been 

 a subterranean outlet, which was capable of draining 

 the water out of the greater portion of the basin of 

 Loch Lomond. Such subterranean outlets at the 

 junction of two or more breaks are not uncommon 

 among similar rocks in the hills of larconnaught, 

 Ireland, where the " fault-rock " is of an incoherent 

 nature or open structure; or when an internal fissure 

 has not been filled with minerals. Such a theory as 

 this would necessitate that all the rock debris taken 

 out of the valley which may have been more or less 

 in accordance with the size of the original fissures 

 due to the shrinkage of the rocks was carried away 

 through the subterranean passages ; now we have 

 previously shown, when describing the subterranean 

 drainage in the neighbourhood of Gort, County Gal- 

 way, 1 that a considerable amount of detritus, and 

 even blocks of stones, may be carried away through 

 such passages. This theory, however, does not appear 

 satisfactory; for although in most lake-basins, especi- 



1 " Memoirs of the Geological Survey, Ireland," Ex. sheets 115, 116. : 



