Mason v. Eldred et al. was a case heard by the United States Supreme Court in 1867. The case involved a dispute between two parties over a contract for the sale of a parcel of land in the state of Illinois. The plaintiff, Mason, had entered into a contract with the defendants, Eldred et al., to purchase the land for a certain sum of money. However, the defendants had failed to deliver the deed to the land as promised, and Mason sued for breach of contract. The Supreme Court held that the contract was valid and enforceable, and that the defendants were liable for damages. The Court also held that the defendants had breached the contract by failing to deliver the deed, and that Mason was entitled to recover the purchase price of the land, plus interest and costs. The Court further held that the defendants were liable for any damages caused by their breach of contract. In conclusion, the Supreme Court held that Mason was entitled to recover the purchase price of the land, plus interest and costs, and that the defendants were liable for any damages caused by their breach of contract.

In Mason v. Eldred et al., the Supreme Court was asked to decide whether a state law that allowed for the sale of public lands in Iowa was constitutional. The majority opinion held that it was, but Justice Field dissented from this decision. He argued that Congress had exclusive authority over public lands and could not be divested of this power by any state legislature or court. He further noted that while states may have some control over their own internal affairs, they cannot pass laws which interfere with federal powers granted by the Constitution or acts of Congress passed pursuant thereto. As such, he concluded that the Iowa statute allowing for sales of public land violated both congressional authority and the Supremacy Clause of Article VI in the U.S Constitution because it interfered with Congressional power to dispose of its property as it saw fit without interference from any other branch or level government