The Post-tertiary or Quaternary division comprises the concluding chapters of geological history. The Pleistocene (pleistos, most, and kainos, recent) contains no extinct species of shells, but a number of extinct mammalia. In the Recent deposits all the species of animals and plants are living. The Tertiary and Quaternary formations are partly of marine and partly of terrestrial and fresh-water origin. At the close of the Tertiary period the 'glacial epoch' of Pleistocene times began, and the British Islands and a large part of northern Europe and North America were then cased in snow and ice. Traces of glacial conditions have also been met with in the Eocene and Miocene. The evidence furnished by Palæozoic and Mesozoic formations points chiefly to mild, genial, and sometimes tropical conditions. But traces of ice action are occasionally noted (namely, in the Silurian, Old Red Sandstone, Carboniferous, Permian, and Cretaceous formations), pointing, perhaps, in some of the cases, to former alternations of cold and warm periods. Indeed, the belief is now gaining ground, that the so-called glacial epoch of Pleistocene times was not one long continuous age of ice, but rather consisted of an alternation of warm and cold periods. And it is not improbable, but highly likely, that similar alternations of climate have happened during every period of great eccentricity of the earth's orbit.
QUESTIONS.
Section 1. What is Geology?
2. Define the term rock. How many classes of rock are there?
3, 4, 5. Into what groups are the mechanically formed rocks divided? Define the terms conglomerate, sandstone, and shale.
6. What is the nature of the rocks belonging to the Aërial or Eolian group?
7. Give an example of a chemically formed rock.