His chief work is his , , in twelve books. The last book, as hitherto published, is imperfect at the end. The object of the treatise is to shew, in opposition to the universal opinion of astronomers, that the earth is not spherical, but an extended surface. The arguments adduced in proof of such a position are drawn from Scripture, reason, testimony, and the authority of the fathers. Weapons of every kind are employed against the prevailing theory, and the earth is affirmed to be a vast oblong plain, its length from east to west being more than twice its breadth, the whole enclosed by the ocean. The only value of the work consists in the geographical and historical information it contains. Its author describes in general with great accuracy the situation of countries, the manners of their people, their modes of commercial intercourse, the nature and properties of plants and animals, and many other particulars of a like kind, which serve to throw light on the Scriptures. His illustrations, which are far from being methodically arranged, touch upon subjects the most diverse. He speaks, for example, of the locality where the Israelites passed through the Red Sea, their garments in the wilderness, the terrestrial paradise, the epistle to the Hebrews, the birthday of the Lord, the rite of baptism, the catholic epistles, Egyptian hieroglyphics, the state of the Christians in India, their bishops, priests, &c.

But the most curious and interesting piece of antiquarian information relates to that celebrated monument of antiquity which was placed at the entrance of the city Adulite, consisting of a royal seat of white marble consecrated to Mars, with the images of Hercules and Mercury sculptured upon it. On every side of this monument Greek letters were written, and an ample inscription had been added, as has been generally supposed, by Ptolemy II. Euergetes (B. C. 247-222). This was copied by Cosmas, and is given, with notes, in the second book of the Topography. It appears, however, from the researches of Mr. Salt, that Cosmas has made two different inscriptions into one, and that while the first part refers to Ptolemy Euergetes, the second relates to some Ethiopian king, whose conquests are commemorated on the inscription. The author also inserts in the work, in illustration of his sentiments, astronomical figures and tables. We meet too with several passages from writings of the fathers now lost, and fragments of epistles, especially from Athanasius.

Photius (cod. 36) reviewed this production without mentioning the writer's name, probably because it was not in the copy he had before him. He speaks of it under the titles of Χριστιανοῦ Βίβλος, ; the former, as containing the opinion of Christians concerning the earth; the latter, because the first part of the work treats of the tabernacle of Moses and other things described in the Pentateuch. The same writer affirms, that many of Cosmas's narratives are fabulous. The monk, however, relates events as they were commonly received and viewed in his own time. His diction is plain and familiar. So far is it from approaching elegance or elevation, that it is even below mediocrity. He did not aim at pompous or polished phraseology; and in several places he modestly acknowledges that his mode of expression is homely and inelegant.