Nor must it be supposed that these islands of fertility and verdure are always _small_. Some of them are very extensive, and contain a considerable population. There is one called the Great Oasis, which consists of a chain of fertile tracts of about a hundred miles in length. Another, called the Oasis of Siwah, has, in modern times, a population of eight thousand souls. This last is situated not far from the shores of the Mediterranean Sea--at least not very far: perhaps two or three hundred miles--and it was a very celebrated spot in Alexander's day.