OF PHYSICAL GEOGRAPHY. 7 



resting in the 'trough' of the Chalk; Pickering Vale in the 

 hollow of the Oolites ; and a great part of the West Riding in 

 the depression of the Coal tract. 



If we suppose some of the strata which compose the lands of 

 Yorkshire to appear in their original position in the sea bed, 

 they would lie nearly horizontal, and present, in section, the 

 appearance of the lower part of fig. 2, PL II. If next we sup- 

 pose an upheaving forfce to be exerted in the direction marked 



by the arrow t on the same figure, we shall have one of two 



appearances ; either the strata will be uplifted on both sides and 

 bent, as fig. 2, or uplifted on one side and broken, as fig. 1. 



Each of these cases occurs in Yorkshire, the axis of uplifting 

 being on or near to the western boundary of the county ; and 

 hence the eastward or south-eastward slopes of the strata so up- 

 lifted. But the surface is not marked with the uniformity of 

 the slope which belongs to the strata. It is undulated by hills 

 and valleys which cannot be explained by the act of elevation 

 of the strata. These undulations are due to the violent action 

 of the sea upon the rocks as they were raised out of the water ; 

 and to the subsequent effect of the atmosphere, rains, and streams 

 in the thousands of years during which the elements have been 

 warring against them. 



The successive steps by which the originally even and con- 

 tinuous surfaces of the strata have been cut and worn into the 

 irregular forms of hills and valleys, may be understood by de- 

 scriptions on paper, but more completely represented on models, 

 and may be actually and experimentally witnessed on the sands 

 of the sea-shore, or verified by artificial arrangements. Let us 

 attempt an illustration of the process on paper. 



In the section (PI. II. fig. 2) W W is the level of the sea, 

 under which are the strata marked G, a hard rock, as sandstone, 

 S, a softer rock, as shale, and L, a firm limestone. All 

 these rocks are divided by fissures, which have characteristic 

 features in each : in the sandstone they are somewhat irre- 



