169. It would be impossible to over-emphasize the importance of foundations, because the very fact that the foundations are underground and out of sight detracts from the consideration that many will give to the subject. It is probably true that a yielding of the subsoil is responsible for a very large proportion of the structural failures which have occurred. It is also true that many failures of masonry, especially those of arches, are considered as failures of the superstructure, because of breaks occurring in the masonry of the superstructure, which have really been due, however, to a settlement of the foundations, resulting in unexpected stresses in the superstructure. It is also true that the design of foundations is one which calls for the exercise of experience and broad judgment, to be able to interpret correctly such indications as are obtainable as to the real character of the subsoil and its probable resistance to concentrated pressure.