The text appears in Launcelot D. Dowdall's translation of De Mirabilibus Auscultationibus, published in 1909 by the Clarendon Press, Oxford.
In the sea outside the Pillars of Hercules they say that an island was discovered by the Carthaginians, desolate, having wood of every kind, and navigable rivers, and admirable for its fruits besides, but distant several days' voyage from them. But, when the Carthaginians often came to this island because of its fertility, and some even dwelt there, the magistrates of the Carthaginians gave notice that they would punish with death those who should sail to it, and destroyed all the inhabitants, lest they should spread a report about it, or a large number might gather together to the island in their time, get possession of the authority, and destroy the prosperity of the Carthaginians.
This account of a marvellous island out in the Atlantic Ocean comes from Oldfather's 1933 translation of Diororus.
19.1. But now that we have discussed what relates to the islands which lie within the Pillars of Heracles, we shall give an account of those which are in the ocean. For there lies out in the deep off Libya an island of considerable size, and situated as it is in the ocean it is distant from Libya a voyage of a number of days to the west. Its land is fruitful, much of it being mountainous and not a little being a level plain of surpassing beauty. 2. Through it flow navigable rivers which are used for irrigation, and the island contains many parks planted with trees of every variety and gardens in great multitudes which are traversed by streams of sweet water; on it also are private villas of costly construction, and throughout the gardens banqueting houses have been constructed in a setting of flowers, and in them the inhabitants pass their time during the summer season, since the land supplies in abundance everything which contributes to enjoyment and luxury. 3. The mountainous part of the island is covered with dense thickets of great extent and with fruit-trees of every variety, and, inviting men to life among the mountains, it has cozy glens and springs in great number. In a word, this island is well supplied with springs of sweet water which not only makes the use of it enjoyable for those who pass their life there but also contribute to the health and vigour of their bodies. 4. There is also excellent hunting of every manner of beast and wild animal, and the inhabitants, being well supplied with this game at their feasts, lack of nothing which pertains to luxury and extravagance; for in fact the sea which washes the shore of the island contains a multitude of fish, since the character of the ocean is such that it abounds throughout its extent with fish of every variety. 5. And, speaking generally, the climate of the island is so altogether mild that it produces in abundance the fruits of the trees and the other seasonal fruits for the larger part of the year, so that it would appear that the island, because of its exceptional felicity, were a dwelling-place of a race of gods and not of men.
20.1. In ancient times this island remained undiscovered because of its distance from the entire inhabited world, but it was discovered at a later period for the following reason. The Phoenicians, who from ancient times on made voyages continually for purposes of trade, planted many colonies throughout Libya and not a few as well in the western parts of Europe. And since their ventures turned out according to their expectations, they amassed great wealth and essayed to voyage beyond the Pillars of Heracles into the sea which men call the ocean. 2. And, first of all, upon the Strait itself by the Pillars they founded a city on the shores of Europe, and since the land formed a peninsula they called the city Gadeira; in the city they built many works appropriate to the nature of the region, and among them a costly temple of Heracles, and they instituted magnificent sacrifices which were conducted after the manner of the Phoenicians. And it has come to pass that this shrine has been held in an honour beyond the ordinary, both at the time of its building and in comparatively recent days down even to our own lifetime. Also many Romans, distinguished men who have performed great deeds, have offered vows to this god, and these vows they have performed after the completion of their successes. 3. The Phoenicians, then, while exploring the coast outside the Pillars for the reasons we have stated and while sailing along the shore of Libya, were driven by strong winds a great distance out into the ocean. And after being storm-tossed for many days they were carried ashore on the island we mentioned above, and when they had observed its felicity and nature they caused it to be known to all men. 4. Consequently the Tyrrhenians, at the time when they were masters of the sea, purposed to dispatch a colony to it; but the Carthaginians prevented their doing so, partly out of concern lest many inhabitants of Carthage should remove there because of the excellence of the island, and partly in order to have ready in it a place in which to seek refuge against an incalculable turn of fortune, in case some total disaster should overtake Carthage. For it was their thought that, since they were masters of the sea, they would thus be able to move, households and all, to an island which was unknown to their conquerors.