Chapter III of the book is devoted to crystals and minerals, the 'building blocks of rocks' as Frédéric Boulvain describes them. This is the kingdom of crystallography and the author recalls a few basic principles of it, as well as taking a closer look at the physical properties of minerals: 'many of these properties are, in effect, a reflection of their chemical composition and their crystallographic characteristics', stresses the author. Alongside the expected properties (mechanical, magnetic, thermal, etc.) are others with surprising importance: organoleptic properties! Yes, geologists also used their five senses to identify or classify rocks. Taste can help identify minerals which are soluble in water, such as mineral salts, the smell of clays and the feel of talc, for example, but clearly it is there colour which is the most characteristic property of many of them. Of course, and the author spends some time on this, determinative mineralogy is above all the result of two proven methods: X-ray diffraction and optical crystallography? The first consists of sending a beam of X-rays onto a crystal (e.g. a powder), which gives a characteristic diffraction figure of the crystal being studied. The second uses the optical properties of minerals: plates (or thin slides) of rock measuring 30 microns thick are observed under a polarising microscope. The explanation of these identification methods boils down to a simple question: how many different minerals make up the Earth? When we look at plant and animal species the answer is obvious: there are millions. But the mineral world is much less diverse. 'The number of known minerals is around 5,000', explains Boulvain. 'Moreover, every year, we only discover about a hundred new mineral species. The problem of the limitation of the number of minerals is connected to the relative monotony of geological processes. This follows a clearly identified path which leads to the selective concentration of certain chemical elements in various parts of the Earth's crust under precise thermodynamic conditions.' Frédéric Boulvain presents the classes of minerals with more detail for silicates, the class which in itself undoubtedly comprises half of all known minerals.

The Earth's rocks

A change of scale once more from Chapter IV onwards, as the book moves from the examination of minerals to rocks: 'Rocks are natural, coherent or unconsolidated materials, consisting of a combination of minerals. Fossils and debris from other rocks may combine with these minerals.' The study of different types of rock is obviously an important part of the book. The author presents them in three main groups. The first consists of magmatic rocks which result from the cooling and crystallisation of magma. Plutonic rocks are found here, which are formed at great depth, and of course volcanic rocks formed by the cooling of lava from volcanoes. The second group is that of sedimentary rocks which result from the consolidation (diagenesis) of sediments (which are themselves the product of disaggregation or dissolution of rocks). In general, these are deposited in strata and contain fossils; sandstones, clays, limestones, dolomite, etc. Finally, the third group, which is in some ways a synthesis of the first two, includes metamorphic rocks resulting from the modification through heat or pressure of magmatic or sedimentary rocks.