EXHIBIT 10.21

[            ]

U.S. Silica Holdings, Inc.

8490 Progress Drive, Suite 300

Frederick, Maryland 21701

In light of recent court decisions concerning the rights of corporate directors and officers (including directors designated by sponsor shareholder investors) to advancement of expenses and indemnification, Golden Gate Private Equity, Inc. (“Golden Gate”) and U.S. Silica Holdings, Inc. (the “Company”) have agreed to enter into this Letter Agreement (this “Agreement”) to clarify their understandings with respect to certain matters. Capitalized terms not defined elsewhere in this Agreement are used herein as defined in Section 3.

This Agreement clarifies certain rights of (i) Golden Gate; (ii) any Golden Gate Affiliate or other persons or entities providing management, advisory, consulting or other services at the direction or request of Golden Gate or any Golden Gate Affiliate to or for the benefit of the Company or any successors or direct or indirect parents or subsidiaries of the Company (individually, including the Company, a “U.S. Silica Company,” and collectively, including the Company, the “U.S. Silica Companies”); (iii) any Fund; (iv) any persons designated by Golden Gate, any Golden Gate Affiliate or any Fund to serve as a director, officer, board observer, partner, trustee, fiduciary, manager, employee, agent, consultant or advisor, or functional or foreign equivalent of the foregoing, of or to any of the U.S. Silica Companies or of or to any partnership or joint venture of which any U.S. Silica Company is a partner or member (collectively, the “Golden Gate Designees”); and (v) any direct or indirect partners (including general partners), shareholders, members (including managing members), affiliates, controlling persons, subsidiaries, directors, officers, fiduciaries, managers, employees and agents of any of the foregoing (those persons and entities identified in (i), (ii), (iii), (iv) and (v) are herein referred to collectively as the “Indemnitees,” and individually as an “Indemnitee”), whether such right exists pursuant to any Organizational Document or any other agreement or document.

The parties intend that (i) the U.S. Silica Companies are and shall at all times be the indemnitors of first resort with respect to any and all matters for which advancement of expenses and indemnification are provided by the U.S. Silica Companies to or on behalf of any Indemnitee, without regard to the time of any related claims and liabilities or of any act, statement or omission relating thereto, (ii) the U.S. Silica Companies shall advance expenses and/or indemnify each Indemnitee on a primary basis, and (iii) any Indemnitee may be required to seek advancement of expenses and/or indemnification from any other potential source of such advancement or indemnification (including from any other Indemnitee) only if, and to the extent, that the U.S. Silica Companies are legally and/or financially unable to advance expenses to or on behalf of and/or indemnify, as the case may be, such Indemnitee.

In consideration of the mutual agreements herein contained, and other good and valuable consideration - including the agreement of the Golden Gate Designees to serve one or more U.S. Silica Companies (including without limitation on the board of directors or similar governing body thereof) - the receipt and sufficiency of which is hereby acknowledged, all parties intending to be legally bound hereby agree as follows:

1. Company is Primary Indemnitor.

Each of the undersigned U.S. Silica Companies hereby acknowledges and agrees that (a) each of the undersigned U.S. Silica Companies is an indemnitor of first resort; (b) the obligations of the U.S. Silica Companies to each Indemnitee are primary, and any obligations of Golden Gate, any Golden Gate Affiliate or any Fund or other Indemnitee to provide advancement of expenses or indemnification for any Losses incurred by Indemnitee and for which any U.S. Silica Company has agreed (or is otherwise obligated) to indemnify Indemnitee (whether under any Organizational Document or any other agreement or document) are secondary; and (c) if Golden Gate, or any Golden Gate Affiliate, Fund or other Indemnitee, is obligated to pay, or pays, or causes to be paid for any reason, any expense or Loss which any U.S. Silica Company is otherwise obligated (whether under any Organizational Document or other document or agreement) to pay to or on behalf of Indemnitee, then (x) Golden Gate, such Golden Gate Affiliate, Fund or other Indemnitee, as the case may be, shall be fully subrogated to and otherwise succeed to all rights of Indemnitee with respect to such payment, including with respect to rights to claim such amounts from any of the U.S. Silica Companies; and (y) each of the undersigned U.S. Silica Companies shall jointly and severally reimburse, indemnify and hold harmless (or cause one or more other U.S. Silica Companies to reimburse, indemnify and hold harmless) Golden Gate, such Golden Gate Affiliate, Fund or other Indemnitee, as the case may be, for all such payments actually made by such entity or person on behalf of or for the benefit of Indemnitee.

2. Specific Waiver of Subrogation, Contribution, etc.

To the fullest extent not prohibited by applicable law, each of the undersigned U.S. Silica Companies hereby unconditionally and irrevocably waives, relinquishes and releases (and covenants and agrees not to exercise, and to cause each affiliate of any U.S. Silica Company not to exercise), any claims or rights that any U.S. Silica Company may now have or hereafter acquire against any Indemnitee (in any capacity) that arise from or relate to the existence, payment, performance or enforcement of one of the U.S. Silica Companies’ obligations under this Agreement or under any indemnification obligation (whether pursuant to any other contract, any Organizational Document or otherwise), including any right of subrogation, reimbursement, exoneration, contribution or indemnification and any right to participate in any claim or remedy of any Indemnitee against any other Indemnitee, whether such claim, remedy or right arises in equity or under contract, statute, common law or otherwise, including any right to claim, take or receive from any Indemnitee, directly or indirectly, in cash or other property or by set-off or in any other manner, any payment or security or other credit support on account of such claim, remedy or right.

2

3. Definitions. As used in this Agreement, the phrase “including” shall not be deemed to be a term of limitation but rather shall be construed to mean “including, without limitation” and the following terms are used with the meaning set forth herein:

4. Miscellaneous.

3

4

[THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK.]

5

ACKNOWLEDGED AND AGREED TO as of the date of this letter.

SIGNATURE PAGE TO INDEMNITY PRIORITY AGREEMENT

SIGNATURE PAGE TO INDEMNITY PRIORITY AGREEMENT