Estimation of the relative amount of earth and rock excavation is a serious problem to engineers in preparing bids for highway construction. As rock may cost five times as much to remove as earth, reliable estimates of the relative amounts of these two substances that must be cut away may be important factors in the engineer's company obtaining the contract or losing money if it does get the contract. The author is an estimator for a large contracting company in Central California, and this short summary indicates problems in making cost estimates in bidding for contracts. Both economic and geologic factors are involved. The chief geologic factors are depth of weathering, previous experience of the estimator with rocks and soil of similar character, and degree of fracturing and toughness of rock. The engineer needs geologic advice in making his estimates and this article points out things the geologist should consider in endeavouring to help the engineer.