Mishnah Oholot 17

¹One who plows a grave [into a field], behold he makes it a bet haperas. To what extent does he make it [a bet haperas]? For the length of a furrow of a hundred cubits, a space in which four se'ahs [can grow]. Rabbi Yose says: an area of five [se'ahs]. But when on an upward or downward slope: he puts a quarter [of a kav] of vetch seed on the knee of the plough, and the space where [the last] three vetches grow next to each other is a bet peras. Rabbi Yose says: [a bet haperas is only made by a plough going] downwards but not upwards. ²If a person was plowing and struck against a rock or a fence, or if he shook the plowshare, he only makes a bet peras up until that spot. Rabbi Elazar says: one bet peras can form another bet peras. Rabbi Joshua says: sometimes it can, but at other times it cannot. How so? If he plowed for half a furrow's length and then returned and plowed a [further] half, or similarly [if he plowed] to the side, he makes a bet peras. If he plowed a full furrow's length and then returned and plowed from that point beyond, he does not make this a bet peras. ³If a person plows from a pit full of bones, or from a heap of bones, Or from a field in which a grave had been lost, Or in which a grave was subsequently found, Or if he plows a field which was not his own, Or if a non-Jew plowed, he does not make a bet peras. For the rule of bet peras does not apply [even] to Samaritans. ⁴[In the case where] there was a bet peras above a pure field, if rain washed down soil from the bet peras to the pure field, even where this was reddish and the [other soil] turned it white, or where this was white and the other turned it red, this does not make it a bet peras. ⁵[With regard to] a field in which a grave had been lost, and upon which a house had been built with an upper story above it: If the entrance of the upper room was directly above the entrance of the house, the upper story remains clean; But if not the upper story becomes unclean. [With regard to] soil from a bet peras, or soil from a foreign country that came in with vegetables, the pieces of the soil combine together [to transmit impurity if they form a portion] the size of a packing-bag seal, the words of Rabbi Eliezer; But the sages say: there must be one portion of the size of a packing-bag seal. Rabbi Judah says: It happened once that letters came from overseas for the sons of the high priests and they had on them about a se'ah or two se'ahs of seals [of dirt], but the sages were not concerned on account of uncleanness.