dissenting.
Oil shale was patentable under the general mining law from *6741872 until 1920.1 In 1920, Congress enacted the Mineral Leasing Act, 30 U. S. C. § 181 et seq. That legislation withdrew oil shale and certain other minerals from the general mining law, but preserved “valid claims existent at date of the passage of this Act and thereafter maintained in compliance with the laws under which initiated, which claims maybe perfected under such laws, including discovery.” Act of Feb. 25,1920, ch. 85, § 37, 41 Stat. 451, as amended, 30 U. S. C. § 193.
The question presented in this case is whether oil shale claims brought under this saving clause of the Mineral Leasing Act must satisfy the usual standards of patentability, or instead may be patented through the use of a “discovery” standard different from that which generally applies. The Court’s answer is that a different and more relaxed standard is applicable. I disagree. Since I believe that pre-1920 oil shale claims must fulfill the then firmly established requirements of patentability for all valuable minerals under the general mining law, I respectfully dissent from the opinion and judgment of the Court.
A
There is not one shred of evidence that Congress enacted the saving clause of the Mineral Leasing Act with the purpose of exempting oil shale claims from the usual requirements of patentability. On its face, the 1920 version of the *675provision applied with identical effect to “coal, phosphate, sodium, oil, oil shale, and gas,” and required that all outstanding valid claims to such minerals meet the existing standards of the mining law in order to be perfected.
Nothing in the Act’s legislative history suggests anything to the contrary. Descriptions by legislators of the saving clause drew no distinction between oil shale and other covered claims. See, e. g., 59 Cong. Rec. 2711-2712 (1920) (Rep. Taylor); 58 Cong. Rec. 7780-7781 (1919).2 In the face of conflicting evidence on the subject, Congress may well have thought that many oil shale claims would meet the traditional criteria of patentability. But it did not accord such claims any special legislative treatment.
Equally unambiguous are the Instructions which the Secretary of the Interior published three months after passage of the Act. These expressly stated:
“[L]ands valuable on account '[of oil shale] must be held to have been subject to valid location and appropriation under the placer mining laws, to the same extent and subject to the same provisions and conditions as if valuable on account of oil or gas. Entries and applications for patent for oil shale placer claims will, therefore, be adjudicated ... in accordance with the same legal provisions and with reference to the same requirements and limitations as are applicable to oil and gas placers.” 47 L. D. 548, 551 (1920) (emphasis added).
*676Such a contemporaneous construction of the statute by the agency charged with its application is entitled to substantial weight. See United States v. National Assn. of Securities Dealers, 422 U. S. 694, 719; Udall v. Tallman, 380 U. S. 1, 16.
B
The saving clause of the Mineral Leasing Act thus directs that the validity of all claims brought thereunder — including those relating to oil shale — must be judged according to the general criteria of patentability that were established in the mining law as of 1920. And I am convinced that nothing that Congress has done since 1920 can be read to have modified this mandate.
The Court points to congressional committee hearings that were held in 1931 on the Secretary’s 1927 Freeman v. Summers decision, and notes that there resulted from this inquiry no legislative rejection of the Department’s then prevailing generous treatment of oil shale claims. But of far greater significance, in my opinion, is the fact that not a single remark by a Senator or Representative, let alone by a congressional committee, can be found approving the liberal standard enunciated in Freeman v. Summers, 52 L. D. 201, even though such a statement could not, in any event, have overridden the plain meaning of the saving clause of the Mineral Leasing Act. See TVA v. Hill, 437 U. S. 153, 191-193; SEC v. Sloan, 436 U. S. 103, 121.
The Court purports to find support for its position in legislation enacted by Congress in 1956. But that legislation dealt with the totally unrelated problem of competing surface and mineral estates, and has nothing to do with the question at issue here. See Pub. L. 743, 70 Stat. 592; S. Rep. No. 2524, 84th Cong., 2d Sess. (1956); H. R. Rep. No. 2198, 84th Cong., 2d Sess. (1956).
The only reasonable inference that can be drawn from the events of 1931 and 1956 is that on those two occasions, as in 1920, Congress declined to assume that every pre-1920 oil *677shale claim would turn out to be unpatentable. It seems to me wholly fallacious to interpret these indications of caution as a congressional intent to exempt oil shale claims from longstanding principles of patentability.
C
The respondents’ patent applications were, I think, quite properly rejected at the administrative level for the simple reason that they failed to satisfy the requirements of the general mining law as of 1920., By 1920, the law, was clear that a mineral land patent could issue only when the applicant had made a “discovery” of a “valuable mineral deposit.” Union Oil Co. v. Smith, 249 U. S. 337, 346 (1919). Through departmental and judicial decisions, it had been further established that a “discovery” occurs only when minerals are found in such quantity and quality as to justify a prudent man to expend his labor and means with a reasonable prospect of success in developing a valuable mine. Chisman v. Miller, 197 U. S. 313, 321-323 (1905); H. H. Yard, 38 L. D. 59, 70 (1909); Castle v. Womble, 19 L. D. 455, 457 (1894). See Cameron v. United States, 252 U. S. 450, 459 (1920); Casey v. Northern Pacific R. Co., 15 L. D. 439, 440 (1892).
Of controlling significance here is the fact that, by 1920, two refinements of this “prudent man test” had occurred. First, it was clear that, although the patent applicant did not have to demonstrate that his mining efforts would definitely yield some profit,3 he at least had to show that they probably would. Cataract Gold Mining Co., 43 L. D. 248, 254 (1914). See Cole v. Ralph, 252 U. S. 286, 299 (1920); Cameron v. United States, supra, at 459; United States v. Iron Silver Mining Co., 128 U. S. 673, 684 (1884).4 Second, *678this required showing of probable profitability had to rest primarily on presently demonstrable, not speculative, fact. See Davis’s Administrator v. Weibbold, 139 U. S. 507, 521-524 (1891); Castle v. Womble, supra, at 457 (“the requirement relating to discovery refers to present facts, and not to the probabilities of the future”); Casey v. Northern Pacific R. Co., supra, at 440; Winters v. Bliss, 14 L. D. 59, 62 (1892). Thus, the applicant could not satisfy the applicable standard by pointing to such highly uncertain future events as market changes or technological advances in an attempt to demonstrate a reasonable prospect of success.
Each of these principles had developed rather naturally out of the “prudent man” rule of Castle v. Womble, supra. For land to be deemed “valuable” for mining purposes, and for a prudent man to decide to expend his time and money in developing a mine upon that land, it was quite rational to require a showing of a reasonable prospect that the mine would yield a profit. See Cataract Gold Mining Co., supra, at 254. The Court is simply mistaken in suggesting that the general mining law was in any way otherwise in 1920.
D
With respect to the oil shale deposits at issue in this case, the Board of Land Appeals found that they “never have been a valuable mineral deposit within the meaning of the general *679mining law.” The Board based this conclusion on the following factual findings:
“First, as a historical fact, the commercial production of oil from oil shale has never been competitive with the liquid petroleum industry. Second, the hypothetical studies [in the record] at best confirm that the commercial exploitation of oil shale would not be competitive with the liquid petroleum industry. Third, without exception, every oil shale operation that has been attempted in this country has failed to show profitable production. Fourth, [the respondents] have held these claims for half a century without attempting to exploit them.
“It is. unlikely that any oil shale operation could have operated at a profit at the time these claims were located or at any time up to and including the time of these contest proceedings. . . .
“In order for a commercially profitable operation to come into being there must be either a dramatic improvement in the technology or an alteration of the economic forces which have always operated in this country to prevent the commercial production of oil shale.”
The Court of Appeals seemed to acknowledge that these findings were supported by substantial evidence, 591 F. 2d '597, 598-599, but thought that a different standard of patent-ability is applicable to oil shale claims, and today this Court agrees with that view.
For the reasons stated, I do not agree. Accordingly, I would set aside the judgment of the Court of Appeals.
Rev. Stat. § 2319 et seq., as amended, 30 U. S. C. § 22 et seq. See Union Oil Co. v. Smith, 249 U. S. 337, 345-346.
The Court’s discussion of a 1919 attempt to substitute “deposits in paying quantities” for “valuable mineral” in a provision of the prospective Mineral Leasing Act, and Representative Sinott’s response thereto, see ante, at 663-664, has absolutely nothing to do with the issue at hand. The attempted substitution concerned a provision of the prospective Act that set out the circumstances under which exploratory permits would be allowed for oil and gas deposits under the new leasing scheme. See 58 Cong. Rec. 7536-7537 (1919). Thus, the legislative discussion quoted by the Court did not involve oil shale, the requirements of the general mining law, or the Act’s saving clause. See id., at 7780-7781.
See East Tintic Consolidated Mining Co., 43 L. D. 79, 81-82 (1914).
See also Royal K. Placer, 13 L. D. 86, 89-90 (1891); Tinkham v. McCaffrey, 13 L. D. 517, 518 (1891). The authorities cited by the Court, ante, at 664, do not support a contrary rule. They state that an applicant *678for a mineral patent need not establish with certainty that a paying mine exists or can be developed on his land, but they do not in any way reject the rule of Castle v. Womble, 19 L. D. 455, 457 (1894), that the applicant must show that there exists a “reasonable prospect of success” in his developing a profitable mine. See Cascaden v. Bartolis, 146 F. 739, 741-742 (CA9 1906); United States v. Ohio Oil Co., 240 F. 996, 998-1004 (Wyo. 1916); Montana Cent. R. Co. v. Migeon, 68 F. 811, 814-818 (CC Mont. 1895); Book v. Justice Mining Co., 58 F. 106, 120, 123-125 (CC Nev. 1893); Madison v. Octave Oil Co., 154 Cal. 768, 771-772, 99 P. 176, 178 (1908); 2 C. Lindley, American Law Relating to Mines and Mineral Lands § 336, pp. 768-773 (3d ed. 1914).