of immense magnitude, the nature of the production of which he has well described and explained in the "Proceedings, Geological Society, London, 1872." The relations between meteorological phenomena proper and Earthquakes have always been a subject of popular belief and superstition. This was here carefully discussed, and with the result of disproving any connection, or, if any, but of an indirect nature. I also, to some extent, towards the end of this Report, discussed the question of the possible nature of the _impulse itself_ which originates the shock; I showed that it must be of the nature of a blow, and ventured to offer _conjecturally_ five possible causes of the impulse: 1. Sudden fractures of rock, resulting from the steady and slow increase of elevatory pressure. 2. Sudden evolution (under special conditions) of steam. 3. Sudden condensation of steam, also under special conditions. 4. Sudden dislocations in the rocky crust of the earth, through pressure acting in any direction. 5. Occasionally through the recoil due to explosive effects at volcanic foci (p. 79-80). The first and last of these I am, through subsequent light, disposed now to withdraw or greatly to modify. The first, the supposed "_snap and jar_, occasioned by the sudden and violent rupture of solid rock masses," to which Mr. Scrope, in his very admirable work on Volcanoes, is disposed to refer the impulse of earthquake shocks (Scrope, 2nd edit., p. 294), I believe may be proved on acknowledged physical principles--when applied to the known elasticities and extensibilities of rocks, and keeping in view the small thicknesses fractured _at the same instant_--to be capable of only the most insignificant impulsive effects; and if we also take into consideration that strata, if so fractured, are necessarily not _free_, but surrounded by others above and below, any such impulsive effect emanating from fracture may be held as non-existent or impossible. In the statement of his views which follows, and in objecting to my second and third possible causes (p. 295-296, headed "Objections to Mallet's Theory"), Mr. Scrope appears to me to have fallen into the error of assuming that the nature of the _impulse_, or the cause producing it, forms any part of "my theory of earthquake movement," or in anywise affects it. I carefully guarded against this in the original Paper ("Transactions, Royal Irish Academy," Vol. XXI., p. 60, and again, p. 97), when I stated "it is quite immaterial to the truth of my theory of earthquake motion what view be adopted, or what mechanism be assigned, to account for the original impulse."