100 THE GEOLOGIST. 



off in this case as the others from a knowledge of the rocks beneath 

 that spot. 



LiGN. 16.— Strata of equal thickness deposited at the same angle on an old 

 uueven sea-bed. 



The amount of strata at this given point, a}^, depends on the d^e'gt'h of 

 the ancient sea-led at the period when the uppermost rocks there were 

 deposited, as each of these three diagrams will show. Yet it is from 

 this mode of calculation, by taking the thickness of the strata at their 

 outcrops, and imaginarily piling them one on the other, that we are 

 said to be cognizant of the structure and condition of ten miles of the 

 earth's crust. 



Our ideal section (page 93) — based, however, on truth and facts — ex- 

 hibits the first recognizable sediments, the Harlech grits and Longmynd 

 rocks (No. 1) belonging to a more remote stage of our world than the great 

 accumulations of the Silurian slates, grits, and conglomerates which 

 succeed them. "We perceive the evidence of this in their unconformable 

 infra- position, the Silurian strata abutting against the slopes of these 

 ''bottom-rocks," as these last do against the primordial gneiss banks; 

 and as the sedimentary beds rise higher and higher, they show how 

 the primitive land must have been gradually and slowly submerged 

 beneath the waves for the accumulating deposits to have been spread out 

 alove and above each other. 



Those bottom-rocks" might not have been dry land before the 

 succeeding Silurian sediments were formed — though possibly tUey had 

 been elevated, and were, — but that they went down and down, deeper 

 and deeper, is certain from the tens of thousands of feet of solid earth, 

 with the innumerable lifeless forms they enclose, now piled above 

 them. The old authors believed the sea to rise and fall and change its 

 level, and regarded the eminences of the land as the types of age and 

 stability. "As old and unchangeable as the hills," ''solid as the moun- 



