Opinion of the Court. 234 U. S. Johnson, 116 U. S. 48; Duluth & Iron Range Railroad Co. v. Roy, 173 U. S. 587; Svor v. Morris, 227 U. S. 524). On the contrary, the suit is one wherein rights asserted under a patent are called in question by parties whose only claim to the land was initiated more than fourteen years after the date of the patent. As the fifth question has been presented in separate briefs and the occasion for considering the other questions turns upon the answer to it, we take it up first. It is: "Is petroleum or mineral oil within the meaning of the term 'mineral' as it was used in said acts of Congress reserving mineral land from the railroad land grants?" This granting act, like several others of that period, expressly excluded from its operation "all mineral lands" other than iron and coal lands. No attempt was made at defining "mineral lands," and doubtless the ordinary or popular signification of that term was intended. Apparently it was used in a sense which, if not restricted, would embrace iron and coal lands, else care hardly would have been taken to declare that it should not include them. This was deemed a reasonable inference in Northern Pacific Railway Co. v. Soderberg, 188 U. S. 526, where a contention that it embraced only metalliferous lands was rejected. The question there was, whether it included lands containing valuable bodies of granite, and the holding was that it did. While avoiding an exact definition, the court was of opinion that it comprehended all lands "chiefly valuable for their deposits of a mineral character, which are useful in the arts or valuable for purposes of manufacture." Petroleum has long been popularly regarded as a mineral oil. As its derivation indicates, the word means "rock oil," an oily substance so named because found naturally oozing from crevices in rocks. Its existence in this country was known from very early times, and when this and other railroad land grants, containing an exception of mineral lands, were made, the extraction of oil from its natural reservoir in subterranean rocks had come to be a promising industry and was extending over an increasing area through discoveries of new oil fields. An official report laid before Congress a few months before this grant was made showed that the daily output of the oil wells in Pennsylvania, Ohio, West Virginia, and Kentucky was 12,000 barrels. H. R. Ex. Doc. No. 51, 39th Cong., 1st Sess. In the same year the Supreme Court. of Pennsylvania, in disposing of an oil-land controversy, not only treated the oil as a mineral but spoke of the work of extracting it from the containing rocks as "mining for oil," and, in concluding the opinion, said: "Until our scientific knowledge on the subject is increased, this is the light in which the courts will be likely to regard this valuable production of the earth." Funk v. Haldeman, 53 Pa. St. 229. And in another case that court said: "It is a mineral substance obtained from the earth by a process of mining, and lands from which it is obtained may with propriety be called mining lands." Gill v. Weston, 110 Pa. St. 312, 317. Its mineral character has also been affirmed by the courts of other States. Williamson v. Jones, 39 W. Va. 231, 256; Kelley v. Ohio Oil Co., 57 Oh. St. 317, 328; Murray v. Allred, 100 Tennessee, 100; Wagner v. Mallory, 169 N. Y. 501, 505. Congress at different times has spoken of it as a mineral (15 Stat. 58, 59, c. 41, § 1; Id. 125, 167, c. 186, § 109; 29 Stat. 526, c. 216; 32 Stat. 691, 702, c. 1369, § 42; 36 Stat. 847, c. 421), and this court did so in Ohio Oil Co. v. Indiana, 177 U. S. 190, 202. In the legislation of Congress the term "mineral lands" is not confined to railroad land grants. It occurs in the mining laws, in an excepting clause in the homestead law, and in like clauses in other public-land laws. Evidently it has the same meaning in all. The administration of these laws has rested with the Land Department, and there Opinion of the Court. 234 U. S. fore its course of action in respect of oil-bearing lands— whether it has held them to be mineral or otherwise requires to be noticed. The various mining circulars, instructions and decisions, as published from time to time, show that the matter probably was not considered prior to the first mining circular, July 15, 1873, but that since then the Department has regarded petroleum as a mineral and has treated lands chiefly valuable therefor as mineral lands.1 With a single exception, the rulings have been uniform, and lands of great value have passed into private ownership under them. The single exception is the case of Union Oil Co., 23 L. D. 222, 226, decided August 27, 1896, which was revoked on a motion for review November 6, 1897, 25 L. D. 351. It appears from the later decision that action upon other pending cases turning upon the same question had been suspended in the meantime, so, practically speaking, there has been no break in the Department's rulings. The case of Union Oil Company presented a controversy between that company and the Southern Pacific Railroad Company over a tract of land in California, the former claiming under a placer mining claim and insisting that the land was chiefly valuable for petroleum and therefore mineral, and the latter seeking a patent under its land grant and insisting that the land, even if chiefly valuable for petroleum, was not mineral. In the original decision the Secretary of the Interior held that the word "mineral" embraced only "the more precious metals," such as "gold, silver, cinnabar, etc.," but on the rehearing this view was rejected and the prior rulings holding petroleum to be a mineral 1 Circular July 15, 1873, Copp's Mineral Lands, 61; Letter of Commissioner Burdett, January 30, 1875, Sickles' Mining Laws, 491; Marwell v. Brierly, 10 Copp's L. O. 50; Instructions January 30, 1883, 1 L. D. 572; Roberts v. Jepson, 4 L. D. 60; Piru Oil Co., 16 L. D. 117; Union Oil Co., 25 L. D. 351; McQuiddy v. California, 29 L. D. 181; Tulare Oil Co. v. Southern Pacific Railroad Co., 29 L. D. 269. were reaffirmed and applied, the railroad company's application for a patent being denied. Notwithstanding these persuasive considerations for now regarding petroleum lands as mineral lands within the meaning of the excepting clause in the granting act, we are asked to give effect to the strictly scientific view that petroleum is a resultant of the decomposition of organic matter under certain conditions of temperature and pressure and therefore is not a mineral. As we understand it, scientists are not in full accord upon this point, some ascribing to petroleum an inorganic origin. Encyclopædia Britannica, 11th ed., Vol. 21, p. 318. But, passing this seeming divergence in opinion and assuming that when subjected to a strictly scientific test petroleum is not a mineral, we think that is not the test contemplated by the statute. It was dealing with a practical subject in a practical way, and we think it used the words "mineral lands," and intended that they should be applied, in their ordinary and popular sense. In that sense, as before indicated, they embrace lands chiefly valuable for petroleum. Our answer to the fifth question must therefore be in the affirmative. The other questions are so closely related one to another and turn so largely upon principles of general application to controversies arising out of the public-land laws, including railroad land grants, that it seems the better course to consider them in a general way in connection with those principles, and then to come to the specific answers to be given to them separately. We first notice a contention advanced on the part of the mineral claimants, to the effect that the grant to the railroad company was merely a gift from the United States, and should be construed and applied accordingly. The granting act not only does not support the contention but refutes it. The act did not follow the building of the road but preceded it. Instead of giving a gratuitous reward for something already done, the act made a proposal to the company to the effect that if the latter would locate, construct and put into operation a designated line of railroad, patents would be issued to the company confirming in it the right and title to the public lands falling within the descriptive terms of the grant. The purpose was to bring about the construction of the road, with the resulting advantages to the Government and the public, and to that end provision was made for compensating the company, if it should do the work, by patenting to it the lands indicated. The company was at liberty to accept or reject the proposal. It accepted in the mode contemplated by the act, and thereby the parties were brought into such contractual relations that the terms of the proposal became obligatory on both. Menotti v. Dillon, 167 U. S. 703, 721. And when, by constructing the road and putting it in operation, the company performed its part of the contract, it became entitled to performance by the Government. In other words, it earned the right to the lands described. Of course, any ambiguity or uncertainty in the terms employed should be resolved in favor of the Government, but the grant should not be treated as a mere gift. Two distinct land grants were made to the Southern Pacific Railroad Company, one on behalf of the construction of a main line, and the other (act March 3, 1871, 16 Stat. 573, 579, c. 122, § 23) on behalf of a branch line. We are not here concerned with the latter. The former was made by the act of July 27, 1866, 14 Stat. 292, c. 278. That act first made provision for the construction of a line of railroad, by the Atlantic & Pacific Railroad Company, from Springfield, Missouri, westward through northern Arizona to the Pacific Ocean, and by its third and fourth sections made the following grant of public lands to that company: |