The Cambridge History of India, Vol. I: Ancient India v1.c15.156

v1.c15.156

The Himālayas form an effective barrier against direct invasions from the north: the exceedingly toilsome passes in their centre are traversed only by a few patient traders or adventurous explorers. But at the western and eastern extremities, river valleys and more practicable mountain passes afford easier means of access. Through these gateways swarms of nomads and conquering armies, from the direction of Persia on the one hand and from the direction of China on the other, have poured into India from time immemorial.