48 STRATIGRAPHICAL GEOLOGY 



felspathic sandstones and sandy shales ; moreover there is reason 

 to believe that they are divisible into four zones, the first and 

 third being purely arenaceous, the second and fourth pelitic or 

 argillaceous. Again, there i& evidence that where they are in 

 contact with the Hebridean there is an unconformity between the 

 two. Thus there must have been a striking resemblance between 

 the Torridonian and the Moine schists in their original condition. 



The idea that the Moine schists and flags are only Torridonian 

 in an altered form is by no means new, for it was suggested as 

 long ago as 1899 by Sir A. Geikie in the following passage : 12 

 " Probably the Torridon sandstone was largely drawn upon in the 

 process of the manufacture of these schists, and there may also 

 have been Cambrian or even later sediments which underwent 

 the same conversion into a foliated crystalline condition." The 

 progress of investigation has tended to eliminate the latter 

 possibility, but to confirm the first supposition. 



Thus Messrs. Home and Teall, writing of the eastern schists in 

 the memoir on the N.W. Highlands, pointed out that in several 

 cases tracts of Torridon sandstone below and near the thrust-planes 

 show a tendency to break down into schistose grits, and again 

 that the Moine schists which immediately overlie the Moine thrust' 

 are less completely crystalline than those lying farther to the 

 east, similar cataclastic structures being not uncommon in them, so 

 'that "the question has arisen as to whether they represent crystalline 

 schists more or less broken down or sedimentary rocks which are 

 on the way, so to speak, to become Moine schist " (op. cit. p. 600). 



On the other hand, Dr. Horne and Professor Gregory hold that 

 the Moine schists are pre-Grampian, because in several localities the 

 Grampian Series rests directly upon them. Some of these cases are 

 explicable by overthrusts, but in Glen Tilt the Blair Athol lime- 

 stone lies unconformably on an eroded surface of Moine schist. 

 These geologists consequently believe in the existence of four 

 Archaean Systems in the following ascending order (1) Hebridean, 

 (2) Caledonian (i.e. Moine schists), (3) Dalradian ( = Grampian), (4) 

 Torridonian. 



2. Ireland 



The Archaean rocks o.f Ireland deserve a brief notice because 

 they are for the most part a continuation of the Scottish Grampian 

 Series. There are two lar^e areas in Ireland where such rocks 

 come to the surface from beneath the Palaeozoic formations. One of 

 these areas includes the great^; part of Donegal and the western 

 part of Derry with a portion or Tyrone ; the other comprises the 



