100 
THE GEOLOGIST. 
off in this case as the others from a knowledge of the rocks beneath 
that spot. 
IjIGN. 16. — Strata of equal thickness deposited at the same angle on an old 
uneven sea-bed. 
The amount of strata at this given point, a", depends on the depth of 
the ancient sea-bed 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 
above and above each other. 
Those " bottom-rocks " might not have been dry land before the 
succeeding Silurian sediments were formed — though possibly they 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- 
