406 . ELEMENTARY GEOLOGY 
Hence the question of the dawn of life, and the nature 
of the Archean animals, cannot even be conjectured. 
We have no doubt that life existed, and probably in 
considerable variety; but the changes to which these 
rocks have been subjected have so metamorphosed 
them, that all signs of fossils have been destroyed. 
It is possible also, that the earliest animals had no 
durable shells to leave, but were soft-bodied creatures 
which left little impression on the ocean rock. 
PALEOzoIC LIFE 
Sedimentary Strata. — Above the Archean, the rocks, although 
frequently crossed by igneous masses, and in some places changed 
by metamorphism, may be said to be typically sedimentary beds. 
By their characteristic composition and structure, they show their 
origin as deposits in water. Limestone strata, similar to those 
now being formed by the animals of the coral reefs of mid-ocean, 
are found embedded with shales and other forms of sedimentary 
rock. Ancient beds of sand and pebbles are present in many 
parts of the crust; and during all the ages since the Archean, 
these have been laid down, one layer on another, just as they are ~ 
now gathering in the ocean. 
These beds, originally deposited in a horizontal position on the 
shores and the bottom of the ocean, are now found in many places 
raised above the sea-level. Oftentimes they are still in a hori- 
zontal condition, although in many parts of the world, particu- 
larly among mountains, they are tilted and intricately folded. 
This change of position among the rocks of mountains has in 
many places, as for instance in New England, so altered or meta- 
morphosed the sedimentary strata, that they have reached the 
