Mostly why 細 is read じ? In every list I looked up 細 can only be read サイ and 亜細亜 is never referenced as 難読 in the entries (about 細) I looked up. What was the reason to pick 細 for the じ sound? Can we see 細 as じ in other 当て字?
1 Answer
細 is read "xi(シ)" in Chinese language, so only the sound seems to be used in 亜細亜.
Country names are written in kanji in China, so country names written in Kanji in Japan may be affected by China.
Sources: http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1176604926 https://ja.wikipedia.org/wiki/%E5%9B%BD%E5%90%8D%E3%81%AE%E6%BC%A2%E5%AD%97%E8%A1%A8%E8%A8%98%E4%B8%80%E8%A6%A7