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THE CONTENTS OF THE ROCKS. 141 
with wonder on the exquisite carving on the stems of mighty trunks, 
hundreds of feet in height, that once formed forests as dense and impene- 
trable as those of the Amazon. But more, we can behold the trees stand- 
ing on the very places in which they grew and waved their great branches, 
and can trace their roots as they penetrate the soil beneath. We can 
also gather plants of all kinds—reeds, mosses, rushes, seaweeds, and 
beautiful ferns—preserved entire, and spread out on the rock as delicate 
and perfect as in the finest herbarium. These fossil plants have, like the 
- fossil animals, been examined and classified by botanists, and we possess 
elaborate volumes on the botany of the remote ages when these plants 
grew, similar to those on the existing flora of our globe. 
Traces of Natural Operations—But the rocks bear traces of more 
than all this. On them, we can see the very dints of the raindrops of 
these bygone ages, and can calculate the direction and force of the showers 
that impressed them. We can walk over the rippled sands of the old seas, 
just as we can do over those we played on in childhood. We can also 
look on the footprints of primeval birds, as they stalked in the mud of 
their lake or river homes ; or gaze with astonishment on the great foot- 
prints, as large as a man’s hand, of the huge reptiles that waddled among 
the reeds by the great old rivers. We can look into the craters of extinct 
volcanoes, can follow the flow of the destructive lava, and can gather the 
ashes that once illuminated the darkened heavens. We can trace the 
sources of ancient rivers, and dig in the mud brought down from their 
mountain sources; can draw maps of the continents and seas as they 
existed thousands of ages past; can tell where great ocean-currents 
flowed, bearing huge icebergs, that grated the sea-bottom, and left their 
indelible traces on the granite and trap of our present hills ; and can shew 
where mighty glaciers once existed in valleys famed for their beauty, 
where now the genial sun sheds its warmest rays. In short, every element 
in nature, whether of air, river, or ocean, has left its deepest traces on the 
solid crust of our wonderful globe. 
Agencies in the Formation of Rocks. 
We now proceed to inquire into the manner in which rocks have been 
formed. Any explanation must account for all the phenomena, equally of 
composition, structure, arrangement, and contents. We must, for instance, 
explain how some rocks are stratified, and others not; how some are 
horizontal, and others inclined ; and how plants and animals have come 
to be embedded in them so far below the surface. Are there, therefore, 
" any agencies engaged in the formation of rocks at the present time that 
produce effects the same in kind with these older masses? If we find 
