T he deposits of gypsum in nature may be divided into two classes. One of these comprises the gypsums formed by the alteration of beds of limestone. The vapours of solfataras, the sulphuric acid produced by the slow oxidation of moist sulphuretted hydrogen gas (as shown by Dumas), and springs containing free sulphuric acid are the agents which have changed and are still changing carbonate into sulphate of lime. Such springs, containing from to ths of free sulphuric acid (evolved probably from the reaction between sulphate of lime and siliceous matters under the influence of heat at considerable depths), are frequent in Western Canada; and their effects in giving rise to masses of gypsum in the quaternary clays of that region I have long since pointed out (Rep. Geol. Survey of Canada, 1848, p. 155). The gypsums of the second class, which are the more frequent, are met with interstratified with marls, dolomites, and rock-salt, with which they are evidently contemporaneous. Reserving for another occasion the discussion of the theories which have been proposed to account for the origin and association of these substances, I propose to mention some recent observations of mine which serve to throw light upon the question. When a solution of bicarbonate of lime in carbenic-acid water is mingled with one of sulphate of magnesia, there are formed sulphate of lime and bicarbonate of magnesia; so that in presence of an excess of bicarbonate of lime there is obtained a saturated solution of gypsum, from which