EARLY INDIA

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the staircases between the plains and the plateau. On
the north-east the huge upheaval of Tibet, rising to 16,000
feet above sea-level, shuts off the rest of Asia with an
impassable barrier of sterile and stony uplands, bordered
[ndiawards by a vast mountain region which comprises
many complicated minor systems whose central peaks
are the highest in the world. These are the Himalayas.
From the eastern extremity of the central Tibetan upheaval,
 mountain ranges curving southwards determine
the initial direction of the rivers of China, Siam, Burma,
and Assam, and round off the north-eastern borderland
of India with a series of walls as impassable as the solid
block of the Tibetan plateau, On the extreme north,
abutting on the north-west of Kashmir, the Pamirs (well
called the Roof of the World) flank the depression north
of the Tibetan plateau westward, and mark the geographical
 centre from which spring the Kuen Lun, hedging
in Tibet to the north ; the Himalayas dividing Tibet from
India ; the Thian-shan, which are but the south-western
links in the central orographical axis of Asia which
reaches north-east for 4600 miles to the Behring Straits ;
and the Hindu Kush, with its subsidiary ranges, forming
the north-western barrier of India. It is this northwestern
 barrier which is geographically of such importance
 to India, and which demands the most careful
consideration. Mountainous, difficult, rugged, and often
dangerous, it yet, by reason of its inferior altitude, does
not offer an obstacle of such impassability as is presented
by the Pamirs of the north and the Himalayas of the
north-east. It is across the north-west barrier that the
flood of immigration or invasion has almost invariably
advanced.
Next in order to the region of mountains is the
region of depression which lies at ‘its south-eastern foot,
curving northward across the breadth of the peninsula
from the Bay of Bengal to the Arabian Sea, and including
 all the most fertile and densely populated districts
of Hindustan. This is the great silt-formed land of
[ndia, the land of great rivers which flow through the