Owing to the frequent northerly gales, especially in the winter, the arboreal vegetation of the northern region, and indeed of the whole island, is scanty. There are some woods of ilex and Aleppo pines in sheltered places, and the shrub vegetation consists of myrtle, a _Phillirea_ (wild olive?), and three species of _Erica_. The southern region is more sheltered and more fertile. It consists of an undulating tableland cut by profound ravines and sloping from the hills to the southern coast, where it terminates in rocky cliffs. The formation is a good building limestone of Miocene age with nearly horizontal strata. In this southern region the shrubby vegetation consists of a buckthorn (_Rhamnus Alaternus_) and the _lentisco_ (_Pistacia Lentiscus_). But there are few trees, and the ground is excessively stony. In the ravines the vegetation becomes richer and more varied. There are no rivers or streams, and the people are entirely dependent on wells and cisterns for their supply of water. The rocks abound in caves, some natural, but many excavated in prehistoric times. There is one vast stalactitic cave near the western coast, with smaller branch caverns, and several other caves of the same kind on a smaller scale. One of the principal features of interest in Minorca is the number of prehistoric remains scattered over the southern region. There are a few similar remains in Majorca, but they have been used almost entirely for building materials; and in Minorca they are far more numerous and less injured. The primitive inhabitants appear to have been cave-dwellers. The buildings may have belonged to a later period. They have been described by several observers, notably by M. Emile Cartailhac in his ‘Monuments primitifs des Iles Baléares’;[31] but never more clearly, and with more competent knowledge of similar monuments in other parts of the world, than by Dr. Guillemard in his very able paper read before the Cambridge Antiquarian Society. Dr. Guillemard divides the Minorcan prehistoric buildings into four classes: (1) the so-called towns, (2) the _Naus_ or ship-like edifices, (3) the _Taulas_ or _Bilithons_, (4) the _Talayuts_. The towns, really the size of small hamlets, are surrounded by a wall with a megalithic gateway, and sometimes with small towers on the walls, which consist of large blocks of limestone. Inside there are the remains of small square buildings, with underground low and narrow passages or caves.