286 DR. A. GILLIGAN ON THE PETROGRAPHY OF [vol. lxXV, 



The Torridon Sandstone is believed to have come mainly from 

 the north- west, the abundance of microcline making it impossible 

 that it should have been derived for the greater part from the 

 underlying Archaean rocks, while it also contains pebbles of rocks 

 which do not occur in Scotland. If that deduction be correct (as 

 I believe it is) for the derivation of the Torridonian, then it would 

 also appear that here is confirmation of the conclusions already 

 arrived at from other evidence with regard to the source of 

 material of the Millstone Grit. 



Further, it must be mentioned that the Sparagmite of Scandi- 

 navia is a group of red arkoses and sandstones some thousands of 

 feet thick, and is found over a great extent of country north of 

 Christiania. It bears a striking resemblance to the Torridonian 

 of Scotland, and was in all probability derived from the same 

 source. It appears that the chief point of difference between the 

 Torridonian and the Millstone Grit is due to the mode by which 

 these two series of beds were accumulated. It is now agreed that 

 the Torridonian Series represents a continental deposit, thereby 

 accounting for the evidence of wind-action which it yields in the 

 rounding of the grains and the presence of ' dreikanter.' The 

 Millstone Grit, on the other hand, is undoubtedly an ordinary 

 deltaic deposit, having been borne along with great rapidity bj 

 large rivers, of which it represents the coterminous deltas. Some 

 of the beds of Millstone Grit which are stained red, such as the 

 Plumpton and Follifoot Grit near Knaresborough, are sufficiently 

 like the Torridonian to deceive anyone when examined in a hand- 

 specimen, but a lens reveals the better rounding of the grains in 

 the Torridonian. 



If we assume, then, a river fed by streams rising among the 

 mountains to the north, possibly fed in many cases by the melt- 

 waters of glaciers descending from the snow-fields, such a river 

 would receive tributaries on both banks, bringing in material from 

 the Grampian Highlands and Scandinavia, as well as from the 

 Southern Uplands, and possibly of land which occupied what is 

 now the North Sea. These tributaries from Scotland and Scan- 

 dinavia can be traced on the evidence of the black schist from 

 Blair Athol, and on that supplied by the felspar-porphyry which is 

 probably one of the facies of rhomb-porphyiy of the Christiania 

 region described previously. The old river first debouched into 

 the Carboniferous Sea somewhere off the coast of North -Eastern 

 England, laying down the beginnings of its delta, possibly in 

 Lower Carboniferous times. With the first uplift of the Northern 

 Continent the carrying power of this river increased, while its 

 tributaries again became active agents in removing the material as 

 previously suggested. The engrafting of the tributaries upon the 

 larger river would take place as the deltas grew. 



