As stress is tensor, the structure of materials is the first parameter controlling their behaviour. Natural earth's crust materials have to be studied by engineers at three scales, the rock matrix, the rock mass, and the mountain scale. Each scale displays different chief features, classified after the departures from continuity, isotropy and homogeneity. Geometrical defects of the joint surface increase the rock mass strength. On the contrary, geometrical defects of the schistosity or foliation surfaces increase the deformability and decrease the strength. The scattered anisotropy of the mylonite structure does not leave any direction for stability. At the rock matrix scale, void content or porosity is the chief parameter, then the continuity of the solid phase or the amount and strength of the cement between the grains. Rock engineers have to feel naturalists and look at their materials before any test or work.

Mr. Chairman, dear colleagues. It's for me a great honour to address you by the end of this Symposium. Just before beginning, I would like to thank and compliment the Japan Society of Civil Engineers for having prepared all of this synthetic work presented just before, about soft rock engineering in Japan.

I knew well through the International Tunnelling Association meetings that the Japanese engineers used to meet difficult conditions in soft rock tunnelling. And I am preparing myself to visit tomorrow the prestigious Seikan Tunnel. And I was very glad to hear about the other fields of ground engineering in Japan, such as slope engineering and foundation engineering, whatever for dams or other large structures. More than soft rocks, I would like to speak about weaknesses of the materials within our earth's crust. As an engineer involved since thirty-five years in surface and underground works, most of them civil but some of them mining, I have been highly interested in both rock mechanics and geological engineering. (Fig. 1) I can remember having attended both meetings where LS.R.M. and LA.E.G. were to be born, the first one in Salzburg, Austria, 1962, and tile second one in New Delhi, India, 1964. And since I attended or sent papers to more than half of the chief meetings of both societies, it's why I wouldn't miss this opportunity given to me by the Organizing Committee of the Symposium on Weak Rocks to speak about the naturalistic side of our field.

I can not say that I read the whole pre-prints of the Symposium and no more here can I say I hear all the presentations. Therefore, I understand that many authors and speakers brought here substantial material to the theme of my lecture, and I would apologize for not having introduced all of this material into it - possibly some of this material will contradict my thoughts. Anyway, I shall be glad to take advantage of them in the future.

Of course, I am not trying to cover all of the subject, which would need hours of lecture, and delay the farewell party tonight.

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