A model for the evolution of smectite-illite mixed layered minerals during diagenesis has been previously developed as a function of temperature and time variables using data from seven deep wells. Efficient use of this model assumes certain restrictions, the greatest being a strict use of the same methods to determine mineral composition as those used in developing the model. In such a model, implicit in the nature of kinetics, there are certain limits to its use in determining geological events (the goal oi use for any such model). Among these limits is the fact that the upper portion oi a sedimentary section is not very sensitive to differences in thermal gradient while the deeper portions (> 2 000 m) are much more sensitive to gradient. As the reaction is driven by absolute temperature, a high geothermal gradient gives similar temperatures to a low one in the first 1 000 m of burial whereas the differences in absolute temperature are much more significant at greater depth. Early erosion events (those which affect sediments at low absolute temperature) will be significantly ＇＇annealed＇＇ from the memory of a clay mineral reaction series by further burial, whereas late erosion episodes are well recorded in the deeper sediments where erosion changes the absolute temperature oi a rock layer to a great extent. Hence, events which affect the temperature of a rock layer when it lies at great depth are fully recorded in the clay mineral reaction series while those which change the temperature at shallow depth will be poorly recorded. Certain other precautions and limitations to the use of the model are indicated.