Does a contractor have to obtain rock or gravel from a quarry in order to meet the specifications of a construction contract?

In the case of United States v. Johnson (9 Cir.1946) 153 F.2d 846, the specifications with respect to the construction of a highway contained the following paragraph: 'Sources of Supply. Gravel for crushing is available approximately 0.5 mile right of Station 870 and rock for crushing is available approximately 0.3 mile right of station 1239. Unless otherwise specifically approved in writing by the engineer only materials from the above sources shall be used for crushing. Additional filler that may be necessary to meet the required grading shall be obtained rom sources approved in writing by the engineer.' In undertaking the performance of the work under the contract the contractor discovered that there was soft rock beneath the surface of the quarry.