Dinosaurs | Page 3

group of molluscs, or rather molluscoids for they were not true molluscs, less abundant even then than in previous ages and now surviving only in a few rare and little known types such as the lamp-shell (Terebratulina).
Insects. The Insect life of the earlier part of the Age of Reptiles was notable for the absence of all the higher groups and orders, especially those adapted to feed on flowers. There were no butterflies or moths, no bees or wasps or ants although there were plenty of dragonflies, cockroaches, bugs and beetles. But in the latter part of this era, all these higher orders appeared along with the flowering plants and trees.
Plants. The vegetation in the early part of the era was very different both from the gloomy forests of the more ancient Coal Era and from that which prevails today. Cycads, ferns and fern-like plants, coniferous trees, especially related to the modern Araucaria or Norfolk Island Pine, Ginkgos still surviving in China, and huge equisetae or horsetail rushes, still surviving in South American swamps and with dwarfed relatives throughout the world, were the dominant plant types of that era. The flowering plants and deciduous trees had not appeared. But in the latter half of the era these appeared in ever increasing multitudes, displacing the lower types and relegating them to a subordinate position. Unlike the more rapidly changing higher animals these ancient Mesozoic groups of plants have not wholly disappeared, but still survive, mostly in tropical and southern regions or as a scanty remnant in contrast with their once varied and dominant role.
There is every reason to believe that upon the appearance of these higher plants whose flower and fruit afforded a more concentrated and nourishing food, depended largely the evolution of the higher animal life both vertebrate and insect, of the Cenozoic or modern era.
FOOTNOTES:
[Footnote 1: The records of Egypt and Chaldaea extend back at least sixty centuries.]
CHAPTER II.
NORTH AMERICA IN THE AGE OF REPTILES.
ITS GEOGRAPHIC AND CLIMATIC CHANGES.
North America in the Age of Reptiles would have seemed almost as strange to our eyes in its geography as in its animals and plants. The present outlines of its coast, its mountains and valleys, its rivers and lakes, have mostly arisen since that time. Even the more ancient parts of the continent have been profoundly modified through the incessant work of rain and rivers and of the waves, tending to wear down the land surfaces, of volcanic outbursts building them up, and of the more mysterious agencies which raise or depress vast stretches of mountain chains or even the whole area of a continent, and which tend on the whole so far as we can see, to restore or increase the relief of the continents, as the action of the surface waters tends to bring them down to or beneath the sea level.
Alternate Overflow and Emergence of Continents. In a broad way these agencies of elevation and of erosion have caused in their age-long struggle an alternation of periods of overflow and periods of continental emergence during geologic time. During the periods of overflow, great portions of the low-lying parts of the continents were submerged, and formed extensive but comparatively shallow seas. The mountains through long continued erosion were reduced to gentle and uniform slopes of comparatively slight elevation. Their materials were brought down by rivers to the sea-coast, and distributed as sedimentary formations over the shallow interior seas or along the margins of the continents. But this load of sediments, transferred from the dry land to the ocean margins and shallow seas, disturbed the balance of weight (isostasy) which normally keeps the continental platforms above the level of the ocean basins (which as shown by gravity measurement are underlain by materials of higher specific gravity than the continents). In due course of time, when the strain became sufficient, it was readjusted by earth movements of a slowness proportioned to their vastness. These movements while tending upon the whole to raise the continents to or sometimes beyond their former relief, did not reverse the action of erosion agencies in detail, but often produced new lines or areas of high elevation.
[Illustration: Fig. 2.--North America in the Later Cretacic Period. Map outlines after Schuchert.]
Geologic Periods. A geologic period is the record of one of these immense and long continued movements of alternate submergence and elevation of the continents. It begins, therefore, and ends with a time of emergence, and includes a long era of submergence.
These epochs of elevation are accompanied by the development of cold climates at the poles, and elsewhere of arid conditions in the interior of the continents. The epochs of submergence are accompanied by a warm, humid climate, more or less uniform from the equator to the poles.
The earth has very recently, in a geologic sense, passed through an epoch