
102 STRUCTURAL AND FIELD GEOLOGY 
the corals, and certain of their congeners, often begin to 
diminish in size,and even to become somewhat distorted, as 
if the influx of muddy sediment had acted prejudicially upon 
their growth and development. _ 
Shallow-water conditions and proximity of the land are 
often evidenced by trails, burrows, and castings of annelids, 
tracks of crustaceans, etc., footprints of reptiles, amphibians, 
birds, or mammals. Along with these the strata may yield 
more or less well-preserved plants, insect-remains, and other 
relics of land-life. Beds containing such fossils are not 
infrequently estuarine deposits, and often exhibit ripple- 
marks, rain-prints, and sun-cracks. 
Terrestrial Movements deduced from Fossils.—The 
presence of marine fossils in a rock obviously indicates 
oscillations of the sea-level. The appearance, for example, 
in our maritime districts, at various heights above the present 
sea-level, of terraces of sand and gravel, crowded with sea- 
shells of still living species, is proof positive of some recent 
crustal movement—either the land has risen or the sea-floor 
has subsided. Again, the existence at various depths on the 
sea-bottom of peat overlying the stools of trees belonging 
to kinds that still flourish in these islands, is evidence 
sufficient of a recent subsidence of the land. 
Geological Chronology and Fossils.—In many cases 
it is quite impossible to correlate the formations occurring in 
separate regions by means of lithological characters alone. 
Within limited areas these may be reliable, but strata begin 
to change in character as they extend in various directions, 
Limestones, for example, may become gradually more and 
more argillaceous until at last they merge into shales, while 
these last may in their turn eventually pass into or inter- 
osculate with sandstones. Now, unless such changes could 
be followed in continuous open section, we could not possibly 
be sure that certain given beds of limestone, shale, and sand- 
stone were exactly contemporaneous—all laid down on one 
and the same sea-floor. These rocks are so dissimilar that, 
unless we actually traced the connecting passages, we could 
not tell how one was related to another. So far as lithological 
character is concerned, they might each have been formed 
at a different time, But if the separate sections of strata 
