I am led by the analogy of allied problems in mechanics to assume that the resistance of the body of strata varies with some power of its depth, but I am unable to say _what_ power. So far as I am aware, neither mathematical analysis nor experimentation has been directed to the problem in question. According to Rankine “the resistances of flexure of similar cross-sections [of elastic beams] are as their breadths and as the _squares_ of their depths” (“Applied Mechanics”, page 316), and it is possible that the same law applies to the resistances which continuous strata oppose to the uplifts of domes. But it appears more probable that the greater complexity of the strains developed in the formation of domes causes the depth to enter into the formula with a higher power than the second. On the other hand, some allowance should be made for the fact that the elasticity of the resisting strata is imperfect. If we call the power with which the depth enters the formula a, equation 1 becomes _r_ = _d^a_ _c_ _C_{lll}_ (4). and equation 3 becomes _c_ = _d^a_ (_C_{lll}_/_C_{ll}_) (5). It is probable that the true value of _a_ is not less than 2, nor more than 3. Interpreting these equations in the same manner as those applying to deformation by faulting, we reach the following conclusions: 1st. At a given depth beneath the surface, lava injected under a given pressure cannot form a laccolite of less than a certain area. This may be called its _limital area_. 2d. The pressure of injection remaining constant, the limital area of a laccolite is a direct function of its depth beneath the surface. The limital area is greater when the depth is greater, and less when the depth is less. 3d. A laccolite of small volume will not exceed the limital area, but will grow by lifting its cover. If however the volume of intruded lava be great, its own weight becomes a factor in the equilibrium of forces and modifies the distribution of the pressures. As the rock bubble rises, the weight of the contained fluid is progressively subtracted from the pressure against its top, and this proceeds until the upward and lateral pressures become proportional to the resistances which severally oppose them. Further expansion is then both upward and outward.