96 STRUCTURAL AND FIELD GEOLOGY 
only sparse traces are met with in marine formations. It 
is obvious, indeed, that the great majority of land-plants 
and animals must necessarily disappear without leaving any 
trace behind. The surface on which they live is pre-eminently 
a region of disintegration and denudation rather than accumu- 
lation. It is, therefore, only under exceptional circumstances 
that relics of land-life. can be preseryed. Whe sea, onuane 
other hand, is par excellence the region of accumulation. The 
creatures which live and die there are thus much imonre 
likely to be represented. It is simply for this reason that 
the records of marine life are so much more continuous and * 
abundant than those of land-life. Hence relics of land-plants 
and animals are, generally speaking, of relatively less value 
to the geologist for the purpose of comparing and correlating 
separate areas of fossiliferous strata. Nevertheless, some of 
the most absorbingly interesting and fascinating chapters in 
the world’s history have been rescued from terrestrial and 
lacustrine formations. It must also be noted that in certain 
cases it has been possible to correlate widely separated areas 
of terrestrial and freshwater deposits by means of their 
fossils. This holds specially true for certain systems and 
stages, as in the case of the Coal-measures, the coals and 
lignites of later age, and the estuarine and freshwater deposits 
of Secondary, Tertiary, and more recent times. 
A glance at the several great phyla of the animal kingdom will serve 
to show the relative importance to geologists of marine organic remains. 
Protozoana—Among these lowly organised forms are many which 
possess calcareous or siliceous hard parts. Members of this phylum 
therefore occur in great abundance in marine formations of all ages. 
Porifera (Sponges).—A few of the living types are of freshwater 
habitat, but the great majority are marine, and the same was the case in 
earlier ages. As many sponges are furnished with a calcareous or a 
siliceous skeleton or framework, they are somewhat common fossils in 
many marine deposits. 
Calenterata.—These also are essentially marine. The phylum 
includes the corals, which are among the most abundant fossils—often 
forming immense sheets and masses of limestone. 
Echinodermata.—This is another great division of marine creatures, 
amongst which are star-fishes, sea-urchins, and stone lilies—the cal- 
careous tests and skeletal remains of which are among the most frequently 
occurring fossils in many formations. 
Annelida (Worms).—These are known as fossils chiefly by their tracks 
and castings—being for the most part soft-bodied creatures, they have 

