4th. There is a limit to upward expansion, dependent on the fact that the pressure due to the combined weight of the laccolite and cover cannot exceed the pressure of the intrusive lava. Regarding the intrusive pressure as constant, it is divisible into three parts, of which one sustains the weight of the cover, also constant; another sustains the weight of the fluid laccolite, and is measured by its thickness or depth; and the third produces deformation. When the sum of the weights of the cover and laccolite equals the total pressure of the intrusive lava, uplift ceases, and the maximum depth or thickness is attained. We may call this the _limital thickness_. With regard to simple laccolites the limit is absolute, but it applies only to the distinct layers of those which are composite; for a composite laccolite, built by successive intrusions at wide intervals of time, may be relieved of part of its load by the erosion of the mound which its expansion causes at the surface of the land. A laccolite formed beneath the bottom of a sea has a greater limital thickness than one formed beneath a land surface; for the superjacent water being displaced and thrust aside, is to that extent subtracted from the load to be lifted. 5th. The laccolite in its formation is constantly solving a problem of “least force”, and its form is the result. Below, above, and on all sides its expansion is resisted, and where the resistance is greatest its contour is least convex. The floor of its chamber is unyielding, and the bottom of the laccolite is flat. The roof and walls alike yield reluctantly to the pressure, but the weight of the lava diminishes its pressure on the roof. Hence the top of the laccolite becomes broadly convex, and its edges acutely. Local accidents excepted, the walls oppose an equal resistance on every side; and the base of the laccolite is rendered circular. The second of the conclusions enunciated above is susceptible of test by observation. By selecting those laccolites of which the dimensions are known with the best degree of approximation, the following table has been formed: │ Formations. │Titles of Laccolites.│Diameters│Means. │ │ │in miles.│