Though basic rocks are not very abundant in the Alps, they are, nevertheless, important on account of the problems they raise. Several large groups can be distinguished : the Mesozoic ophiolites ; the spilites of Permo-Carboniferous or Triassic age belonging to the Glaris Verrucano and the mantel of the Pelvoux; the "green-stones" of the Casanna schists; the basic rocks of the Ivrea and adjacent zones; the Permian porphyrites of the Southern Alps; various lenses of basic rocks in the old cristalline parts of the autochtonous Hercynian massifs or of the Pennine and Austro-Alpine nappes. A small sub-group of Mesozoic ophiolites has escaped metamorphism. Study of this sub-group has revealed the frequent presence of spilitic pillow-lavas in these non-metamorphic zones. The pillows are subject to great variation as regards structure, mineral composition and chemical composition. These differences become apparent when the pillows are examined from the centre outwards towards their periphery. The relation of these spilites with the non-spilitic diabases is still problematic. The same can be said of the relation between the diabases as a whole and the ultra-femic members (serpentines and similar rocks) accompanying them.