То the question of origin of Cossacks and appearance the first Cossacks in Zabaikalie

There is written in Brockhaus and Efron dictionary: «Cossacks are the ones of the Russian Empire heritage and they have got special rights and properties...» There are no exact definition of Cossacks origin and the word «Cossack» as well in contemporary historical literature. History of Zabakalian cossacks numbers about 370 years. The first Russian Cossacks appeared in Zabaikalie in 30th years of XVII century. And the first stockaded town in Zabaikalie was the Barguzinian, which was found in 1648.