284 



DB. 



A. GILLIGAJHT ON THE PETEOOBAPHY OF [vol. lxxv, 



the main direction of the main river will be found in a large 

 proportion of the readings : as in the diagram, north-east to 

 south-west, and north-west to south-east, the main direction being 

 from north to south. Again, the distributaries will have their 



courses altered from time to 



Fig. 3. — Diagram illustrating time, and the current-bedding 



courses of distributary would not therefore be in the 



streams in a great river- same direction in a vertical 



delta. section. Pursuing this line 



of investigation, I have found, 

 in agreement with Sorby, that 

 the prevalent direction is 

 north-east to south-west. 

 Sorby made his observations 

 in South Yorkshire, while 

 mine have been made in Aire- 

 dale, Wharfedale, and Nidder- 

 dale, as well as on outliers of 

 : grit on Ingleborough, Foun- 

 tains Fell, Whernside, Bowland 

 Knotts, Penygent, etc. The 

 best place to make such tests 

 is, of course, on isolated masses 

 of grit such as can be found 

 in plenty on the tops of the hills mentioned, as well as in the 

 low ground in such well-known areas as Plumpton Eocks and the 

 similar masses near Spofforth and Scriven. 



The frequent occurrence of beds of pebbles in such current- 

 bedded grits as those mentioned above is of great interest, and of 

 course they represent the heavy pebbles which have rolled down 

 the slope of an advancing sandbank. 



XII. Comparison with the Tobeidon Sandstone. 



As the investigation proceeded, it became increasingly evident 

 that there was a striking similarity in composition between the 

 Torridon Sandstone of pre-Cambrian age found in the North- West 

 of Scotland and the Millstone Grit of the North of England. 

 I have, therefore, had sections of the Torridon Sandstone pre- 

 pared for comparison, and found a very pronounced agreement in 

 all essential points, the most marked difference being the rounding 

 of the grains in the Torridon Sandstone as compared with the 

 general angularity of the grains in the Millstone Grit. One has 

 only to read the chapter on the petrography of the ' Torridonian 

 Formation ' in the Geological Survey Memoir on the North- West 

 Highlands of Scotland, and to compare it with the petrography 

 of the Millstone Grit, as revealed by this investigation and here 

 recorded, to be convinced of the strong resemblances of the two 

 deposits. 



It is, of course, natural that in the basement-beds of the 

 Torridonian should be found pebbles derived from the underlying 



