Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/754

688 facts which tend to throw light on the probable epochs at which the land has assumed its present configuration.

42. There is evidence of the very recent formation, speak ing geologically, of the great plains of Northern India, of Mesopotamia, and of portions of Central Asia and Siberia. The existence of deposits containing large mam malian remains of the older Pliocene or Miocene divisions of the Tertiary period, has been ascertained at many places on the low lands of British India, in Burmah on the east, along the foot of the Himalaya, and near the Gulf of Cutch on the west, which indicates that very great changes of level and of conditions of surface have taken place in those localities since the later Tertiary epoch. The far wider spread of the older tertiaries proves that far greater alterations have occurred since the Eocene period. Num- mulitic limestones are found from Burmah to Eastern Ben gal ; they are continuous along the flank of the table-land of Baluchistan, through Sinclh to the Himalaya ; they are frequent throughout Persia, Asia Minor, Syria, Egypt, and along the Caspian ; they are found at great elevations in the Himalaya in one locality at more than 16,000 feet above the sea ; all this indicates that over a vast region, which is now dry land, often rising to very great altitudes, there existed a sea area at a period which, geologically speaking, appertains to the more recent epochs of the earth s history. Hence the present configuration of the land must be due to movements of a subsequent date ; and it is thus shown that the vast elevation of the great Tibetan table land is due to changes which are among the latest to which the earth s surface has been subjected. No doubt the Caspian and Aral seas formed part of the ancient ocean which once occupied a great part of what is now Western Asia.

43. The occurrence of a regular succession of the older fossiliferous deposits, from Upper Silurian upwards to the Triassic and Jurassic, and even to the Cretaceous formations, along the line lying to the north of the highest Himalayan ranges, gives ground for the supposition that an ancient sea-coast may have existed along this line ; and that from a still earlier period there was dry land to the south, where the Himalaya now stands. Thus these mountains may have continued, as a chain of some importance, to occupy their present position from a period anterior to the Silurian epoch, while their existing very great elevation is probably due to the disturbances which have taken place since the middle of the Tertiary period.

44. There is no sufficient evidence of the former extent or distribution of the land south of the Himalaya. But from the absence of marine fossiliferous deposits of the older epochs, it has been inferred that the greater part, at least, of the peninsula of India may have been an area of dry land from a very remote time up to the middle of the Tertiary period, during which the great basaltic formations of Central India are supposed to have been thrown up ; a partial submergence then seems to have occurred, followed by a re-elevation, which gave the continent its present form. The fossil remains that have been found indicate the presence, in the Triassic or Permian age, of a great continental area, extending from Europe, and forming a connection across the Indian Ocean between South Africa and the peninsula of India. Grounds also exist for pupposing that the Indian peninsula was formerly united by dry land with Australia, and that, when these conditions prevailed, the peninsula was separated by sea from the rest of Asia. Other evidence of this is to be found in the distribution of the races of men, and of plants and animals.

45. The occurrence of a large area of Silurian and other Palaeozoic rocks in Northern Siberia, extending to the Altai mountains, with an apparent absence of the Mesozoic, has been explained as probably due to the existence of a marine area during the Palaeozoic epoch, at the close cf which it was replaced by an area of land ; this was again submerged in the latest Tertiary period, during which a great part of the country was covered with the diluvial de posits, in which are found the remains of large mammalia.

46. The presence of coal in many parts of China and China. Mongolia is a further indication of great former changes Central of condition of the surface in that part of the continent. 47. Evidence exists of a former far greater extension of Himala glaciers on the Himalaya, possibly at the period during glaciers which the great glacial phenomena of Europe occurred ; but too little is known to enable us to affirm that this in dicates any general period of cold that affected the northern hemisphere as far south as the Himalaya, though the facts are sufficiently striking to suggest such a conclusion. Another explanation of the decrease of Himalayan glaciers is that it was a consequence of the diminution of the fall of snow, consequent on the gradual change of climate which must have followed a gradual transformation of an ocean area into one of dry land. This last-named circumstance would also account for the great changes in the quantity of rain-fall, and in the flow of the rivers, of which there are many indications in Western India, in Persia, and the regions east of the Caspian.

48. A remarkable feature of Asia is the line of volcanic activity which extends along its eastern coast through Kamchatka, the Kurile and Japanese islands, Formosa, and the Philippines, to the Malay archipelago. Thence the line is prolonged to the west, through Java and Sumatra to the Bay of Bengal, where it ends about in lat. 20 N. on the coast of Aracan. To the east and south there also seems to be a connection with the volcanic regions of New Zealand and the islands of the Pacific, and possibly with that of the antarctic land. There is no authentic evidence of active volcanoes in the interior of the continent.

49. Although these facts give an extremely incomplete view of the geological structure of Asia, they will afford some slight idea of the great changes that have taken place throughout all parts of the continent, and serve to convey a warning of the necessity for taking such changes into account, when attempts are made to understand the manner in which animals and plants are distributed on the earth's surface, and how their development may have taken place. The phenomena of living creatures as now observed are the results of various causes which have been in operation through a long period of time. Those causes which still continue to be in action may, in some respects, call for special attention, as determining the precise conditions under which organisations now have to exist, and as being those which now tend to destroy what has been, and to substitute something different in its place. But what re mains necessarily depends on what preceded it, and the knowledge of the conditions and forms of life in the past is an essential element of the proper understanding of life as it now is.

50. Climate is among the most important of the condi- CLIUA tions affecting all forms of life. Under this general term is designated the complicated series of phenomena which arise in the air surrounding any locality, and in the watery vapour diffused through that air. These phenomena, which include the variations of temperature ar.d of moisture, the winds, the rain, and the electrical condition of the air, depend essentially on the action of solar heat on the atmo sphere and the materials of the terrestrial globe. The main conditions which determine the climate of any place, are position on the earth in respect to latitude, elevation above the sea level, and character of the surface in relation to its power of absorbing or radiating heat. Owing to the ex treme mobility of the air, its temperature and the quantity of moisture it contains are further influenced in a very 