72. Introductory.—Although the signs collected together in this chapter can be spoken of as constituting a sign-language, it would be more correct to describe them as idea-grams, each sign conjuring up an idea, modified more or less by: the context of the mute conversation. Thus, the sign for a boomerang may express not only the idea of the article itself, but also, according to the “run of the ‘text,’” the idea of hitting or killing something by its means, or of swapping, manufacturing, or stealing it, &c. The sign of interrogation conjures up the idea of a question, but the nature of the query will depend upon what has gone before or is coming after.

The value of these ideagrams is apparent in the ease of individuals travelling over country the spoken language of whose inhabitants they are ignorant of or only partially acquainted with; also, on the war-path or the chase, where silence is so essential an adjunct to success. For reasons difficult to estimate, their use is strictly enforced on certain special occasions, such as some of the initiation ceremonies (sect. 300, &c.).

I have personally proved the existence of these ideagrams for the whole of North-West-Central Queensland, this area being understood as comprising the various ethnographical districts known as the Boulia, Leichhardt-Selwyn, Cloncurry, Upper Georgina, and Flinders (sects. 2, 46-49); furthermore, on the Middle and Upper Diamantina they are also met with.