The confirmation of the conclusion is as nearly perfect as could have been anticipated. There is no room to doubt that a relation exists between the diameters of laccolites and the depths of their intrusion. Having determined by observation the mean size of the laccolites in the upper and lower zones, as well as the interval which separates the two zones, and knowing approximately the law which binds the size of the laccolite to its depth of intrusion, we can compute the depth of intrusion of each zone. Our result will doubtless have a large probable error, but it will not be entirely without value. Let _x_ represent the thickness in feet of the original cover of the laccolites of the upper zone; and _x_ + 3300 the thickness of the cover of the laccolites of the lower zone. The mean circumference in feet of the upper laccolites is 1.2 π × 5280 = 6336 π. The mean circumference of the lower laccolites is 2.6 π × 5280 = 13728 π. Substituting these values in equation 5, we obtain 6336 π = _x^a_(_C_{lll}_/_C_{ll}_) and 13728 π = (_x_ + 3300)_^a_(_C_{lll}_/_C_{ll}_) Dividing the second equation by the first and reducing, 3300 _x_ = 3300/ª√(¹³⁷²⁸⁄₆₃₃₆ − 1) (6). To obtain a minimum result, assume a = 2; then _x_ = 3300/√(¹³⁷²⁸⁄₆₃₃₆ − 1) = 7000 feet, and _x_ + 3300 = 10300 feet. The summit of the Masuk sandstone is 3,500 feet above the mean level of the upper laccolites; subtracting this from the value of _x_ gives 3,500 feet as the depth of Tertiary strata which overlay the Masuk beds during the epoch of laccolitic intrusion. To obtain a maximum result, assume _a_ = 3; then _x_ = 3300/∛(¹³⁷²⁸⁄₆₃₃₆ − 1) = 11200 feet, _x_ + 3300 = 14500 feet, and the result for the depth of the Tertiary strata is 7,700 feet. I am far from attaching great weight to this speculation in regard to the original depths of the laccolite covers. It is always hazardous to attempt the quantitative discussion of geological problems, for the reason that the conditions are apt to be both complex and imperfectly known; and in this case an uncertainty attaches to the law of relation, as well as to the quantities to which it is applied. Nevertheless after making every allowance there remains a presumption that the cover of the laccolites included some thousands of feet of Tertiary sediments. What evidence we have then, indicates that the epoch of laccolitic intrusion was after the accumulation of deep Tertiary deposits and before the subsequent degradation had made great progress—that it was at or near the close of the epoch of local Tertiary sedimentation.