The occurrence of small quantities of chert and opal, usually in or near exposures of crystalline limestone, but very often in fragments or boulders not quite in sitû, is not unusual in Ceylon. For some time the origin of these siliceous rocks remained obscure; observations made within the present year (1903), however, enable me to give a more detailed account of their mode of occurrence. I have had the advantage of my colleague Mr. James Parsons' company in examining many of the exposures, and have been able to discuss with him the problems raised.