TRANSITION AND SECONDARY STRATA. 89 
ing beings which have been destroyed, it is much more proba- 
ble that they assumed their present form before there was either 
a plant or an animal to be imbedded in them. 
48. Prop. III. The transition and secondary rocks which 
are now found with their strata highly inclined, were deposit- 
ed and consolidated in horizontal beds. 
They bear an intimate resemblance to those accumulations of 
fine clay, sand, and gravel, that are now found in the bottoms of 
lakes and the estuaries of rivers. There are the same alternations 
of coarse and fine materials, indefinitely repeated, and with- 
out any approach to regularity. They are evidently made up 
of what was once a loose mass, destitute of cohesion, and which 
by the infiltration of siliceous, calcareous, or ferruginous matter 
and by other causes has been converted into rocks. Such a mass 
if placed upon a plane that is considerably inclined, will not ar- 
range itself in beds or layers parallel to the surface of the plane, 
but will roll, sink, or slide down to its lowest point. ..In so doing 
it will only obey the most general of all the laws that regulate 
the material world, the law of gravity. 
The case is particularly clear when very thin layers of shells 
or pebbles are interposed between two adjacent strata. It would 
in many instances, have been quite impossible for them to gain 
the positions in which they are found, in any other way, than by 
being strewed uniformly over an horizontal surface, and then 
covered with a stratum of a different kind. These arguments 
will not apply to all the transition and secondary strata : they 
are applicable to the most of them ; and the rest will be found so 
alternating with, or imbedded in those to which it does apply, that 
whatever decision we pass upon the one class, we shall find our- 
selves under the necessity of extending to the other. 
49. Prop. IV. The secondary, transition, and in many 
cases the primitive strata, have been shifted from their origi- 
nal positions into those ivhich they now occupy, by forces which 
have operated since their consolidation. 
This proposition follows as a necessary corollary from the two 
preceding, and is introduced less with a view to a formal proof 
of its truth, than to an enquiry respecting the nature and mode 
of action of the forces by which the changes referred to in it have 
been produced. If these strata were deposited and consolidated 
in horizontal beds, and are now found sometimes almost in a ver- 
tical position, it is plain that a force of some kind has been ap- 
plied to them by which their situation has been changed. It 
may have been the force of gravity, the substance which support- 
ed one of their edges having been removed, and that edge left to 
subside by its own weight ; or the other edge may have been 
lifted up by a force acting from beneath. 
A mill-pond which has been covered with a thick sheet of ice 
during a cold night, and from which a part of the water has been 
drained off, the next day, furnishes a good representation of the 
