Mr. G. Abbott is quite right in saying (Geol. Mag. for June, p. 275) that much work has yet to be done before we understand the formation of flints, and I would add that the work required must include observations on other siliceous concretions such as chert and cherty nodules of all kinds. It is also unquestionably important that we should, if possible, ascertain whether such concretions were formed contemporaneously with the deposits in which they are now found, or whether they are a sort of separation product formed during the consolidation of the rock after upheaval above the sea.