The dolomites which form the Lérins Islands are very hard magnesian limestones dotted with dislocation cracks which can locally release large tabular blocks under the effect of erosion or take the form of large compact horizontal almost impermeable slabs. Rich in dolomite (double carbonate of calcium and magnesium), dolomite has Ca2 + and Mg2 + ions favorable to the development of calcicole and calcifuge plants rare on the tiny Tradelière islet (photo) but more frequent on the neighboring Sainte-Marguerite island.