As they advanced, small wooden forts (or _ostrogs_) were built in suitable places, and became in their turn the starting-posts for new expeditions. The following dates give the best proof of the uncommon rapidity with which the tide of conquest rolled onward to the east. Tomsk was founded in 1604; and the ostrog Jeniseisk, where the neighboring nomads brought their sable skins to market, in 1621. The snow-shoes of the Tunguse, which they sometimes saw ornamented with this costly fur, induced the Cossacks to follow their hordes, of which many had come from the middle and inferior Tunguska, and thus, in 1630, Wassiljew reached the banks of the Lena. In 1636 Jelissei Busa was commissioned to ascend that mighty river, and to impose _jassak_ on all the natives of those quarters. He reached the western mouth of the Lena, and after navigating the sea for twenty-four hours came to the Olekma, which he ascended. In 1638 he discovered the Tana, on whose banks he spent another winter; and in 1639, resuming his voyage eastward by sea, he reached the Tchendoma, and wintering for two years among the Jukahirs, made them also tributary to Russia. In that same year another party of Cossacks crossed the Altai Mountains, and, traversing forests and swamps, arrived at the coasts of the inhospitable Sea of Ochotsk; while a third expedition discovered the Amoor, and built a strong ostrog, called Albasin, on its left bank. The report soon spread that the river rolled over gold-sand, and colonists came flocking to the spot, both to collect these treasures, and to enjoy the fruits of a milder climate and of a more fruitful soil. But the Chinese destroyed the fort in 1680, and carried the garrison prisoners to Peking.