Component of sandstone church (but not the northern quarter) (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

[ba] SILICA (originally an ancient Roman public building, but the term has come to be used for church buildings of a similar shape), without the first two of the eight letters (top quarter, in a down clue = northern quarter). Silicon dioxide, a major component of sandstone.