The men of the West at this time were familiar with many islands. Giraldus Cambrensis takes up the problem of the origin of the islands of the earth and in particular the question of whether they were formed at the time of the Deluge or long before or long after.[1047] His opinion seems to have been that some time after the Flood the lands became replete with animal life and that it was then that the islands came into existence, not violently and suddenly but little by little out of alluvial deposits.[1048] In his emphasis on the gradual and non-catastrophic manner of their formation, Giraldus by hazard enunciates a sound geological doctrine which contrasts favorably with the theories he elsewhere expresses about the violent and sudden appearance of lakes. MIRACULOUS ISLANDS Like mountains and lakes, islands were convenient topographic units to which the medieval mind was wont to attribute fabulous and supernatural qualities. Gervase of Tilbury, for example, describes a certain isle in the sea off the coast of the Kingdom of Arles—Lirniensis, perhaps the Isle de Lérins—where no worms ever are found.[1049] He was unable to decide whether this was due to the extreme holiness of a colony of monks which once dwelt on the island or to some natural peculiarity of the soil. At all events, this reminds us of the tradition about the inability of poisonous reptiles or noxious animals of any kind to exist in Ireland. Giraldus Cambrensis speaks of a floating island in a lake on the summit of Mount Snowdon.[1050] This was said to be blown about by the winds, and shepherds were much startled now and then to see their cattle transported on it from one side of the lake to the other. Giraldus explains this reasonably enough as follows: a portion of the shore had become bound together and made firm and solid as if by ropes formed from the roots of the willow and other plants. After being gradually increased in size by the addition of alluvium it finally broke off. The violent winds prevalent in the vicinity then drove it back and forth over the surface of the lake. This story undoubtedly had a basis of truth, for it is well known that sod floats about on the surface of one of the lakes near Snowdon, but that it could carry cattle upon it is a decided exaggeration. ISLANDS OF ST. BRANDAN