The Authors’ paper presents interesting results from mineralogical tests conducted on the sediments beneath Mexico City. From the title, it could be inferred that the paper would lend some quantitative insight into the reasons for the dis tinctive geotechnical behaviour of the Mexico City sediments. Valuable profiles of the mineralogy, index tests, shear wave velocity, grain-size distribution, and a cone penetration test log are provided; however, it appears that no attempt has been made to quantitatively link the observed “fascinating” behaviours with the presented data if any such links exist. Rather, it is stated that “...results from the present study sug gest that the exceptional properties of Mexico City sediments (i.e., very high water content and plasticity, low shear wave velocity, and remarkable elastic behaviour) are associated with the presence of amorphous material coating the particles.” The authors attribute considerable importance to “amorphous” materials, i.e., noncrystalline oxides of various minerals (principally iron, aluminum, and silicon), in the formation of the Mexican sediments and their observed behaviours. Yet, the processes by which the amorphous material content is determined (Segalen 1968) are chemically aggressive (Bentley 1980; Quigley 1980; Torrance 1995) and could also strip away any carbonate minerals that represent up to 30% of the Mexico City soil mineralogy. The sediments within zone 4 are noted to exhibit considerably different behaviours than those in zones above or below. Of the mineralogical, index-property, and grain-size data presented, it is the differences in calcite, cation exchange capacity, and organic matter which are most characteristic of zone 4 when compared with other zones. It is of interest to compare the Mexico City sediments with illitic sensitive clays in Canada, the United States, and Norway and published data from Fucino, Italy. The Discussers have found interesting correlations between the carbonate content and vertical effective yield stress (preconsolidation pressure) of glacially derived fine-grained soils of New York State and southern Ontario (Boone and Lutenegger 1997). Burghignoli et al. (1991) concluded that carbonates play a role in the observed initial shear modulus, shear wave velocity, and undrained shear strength of the Italian clay and that the influence of carbonate bonding was likely “obliterated” by the large strains involved in penetra tion testing. The sedimentary environment and typical index and mineralogical properties of some of these soils are compared in Table 1. Several differences are striking, namely, (1) The Scandinavian and North American soils were deposited relatively rapidly during retreat of the ice sheets in the last ice age (from approximately 11 000 years before present), whereas those of Italy and Mexico City are reported to have been deposited over a significantly longer period of time (from approximately 64 000 and 600 000 years before present, respectively), i.e., sedimentation rates were likely half an order of magnitude or more slower for the Italian and Mexican deposits. (2) Both the Fucino and Mexico City lacustrine basins were without an outlet for the majority of the inferred sedimentation period. What role might dissolved salts or other minerals within the water body or in the pore water have on the final structure and behaviour of the sediments? Despite many years of research into the effects of salinity on the formation and resultant behaviour of natural sediments, related issues were not discussed in the paper.