From an engineering viewpoint, the ground beneath a site can conveniently be divided into the categories shown in Table 2.1, which are based upon generalizations of its expected behaviour in construction works.
These broad generalizations are, of course, limited in accuracy. But they give the geotechnical engineer a good basis on which to consider, at the start of a project, both the likely construction problems and the methods of investigation which might be used. In practice, it is found that the ground varies continuously beneath a site, and it is not often possible to find sharp transitions from one type of material to another. This then, calls for more refined, systematic, description and classification of soils and rocks.

Les couches crélaciques du Portugal sont très riches en Echinidcs. Les espèces se montrent nombreuses et variées, et plusieurs d’entre elles fort abondantes en individus.
Ce sont les travaux et les recherches de la Section des Travaux Géologiques du Portugal qui en ont fait découvrir la plus grande partie, aussi, jusqu’à présent, elles étaient demeurées à peu près inconnues. Forbes cependant, en 1849 *, a décrit neuf espèces d’Echinides du Portugal. Je n ’ai pas pu jusqu’ici les identifier toutes et il en est parmi elles qui sont probablement jurassiques. M. Delgado a bien voulu me proposer d’étudier et de décrire cette faune échinitique  crétacique encore si peu connue. J’ai été fort heureux de pouvoir accepter, et c’est avec une vive satisfaction que j ’ai commencé l’étude des beaux matériaux qui avaient été mis à ma disposition et me promettaient un champ de travail aussi neuf qu’intéressant. Je désire témoigner toute ma reconnaissance à la Direction de la Section Géologique de ce qu’elle a consenti à me confier ce travail.<...>

Deserts provide an excellent subject for interchange between Earth scientists of differing subdisciplines. The aridity of climate and the sparseness of plants and animals, especially Man, encourage surface processes that produce sedimentary deposits sufficiently distinctive to be recognized unequivocally both throughout the passage of time and from one planet to another. Because of this, and because Lyell's dictum of uniformitarianism is probably most appropriate in desert settings where the impingement of evolving life forms has been least, an exchange between scientists concerned with the living deserts of Earth and beyond, on the one hand, and with ancient Earth counterparts, on the other, is extremely fruitful. Lessons are learned on all sides.
This was the spirit that encouraged 185 participants to gather at a Special Scientific Meeting of the Geological Society of London in May 1986. It is also the underlying rationale for this Special Publication which incorporates a substantial proportion of the contributions made at that Meeting. <...>