Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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For all Shakespeare's great creations are like works of nature, subjects of unexhaustible [sic] study ...
The "prosperity" of 1918-28 has been described as a Pactolian flood of gold, flowing down incessantly, securing million dollar gate-receipts to pugilists and royal incomes to movie-queens, apparently unexhaustible, but ending as abruptly as it began.
This is why if used comptently, these resources could be unexhaustible. Yet, sustainability of any natural resource directly depends upon the scope of investments into its preservation, i.e.