The Fossat or Quicksilver Mine Case was a landmark decision by the United States Supreme Court in 1863. The case involved two rival mining companies, each claiming ownership of a valuable quicksilver mine located in California. At issue was whether one company had acquired title to the mine through prior possession and use, or if it had been preempted by an earlier federal land grant made to another party. In its ruling, the court held that while preemption could be established under certain circumstances, such as when Congress has expressly declared its intention to do so, mere prior possession and use of property did not constitute preemption unless there were other factors present which indicated congressional intent. This decision set an important precedent for future cases involving questions of federal versus state authority over public lands and resources.

The dissenting opinion in the Fossat or Quicksilver Mine Case argued that the majority's decision was contrary to both common law and equity. The dissenters believed that a mine owner should not be able to acquire exclusive rights over minerals found on their property, as this would lead to an unjust enrichment of one party at the expense of another. They further argued that if such a rule were adopted, it could potentially create monopolies and deprive other parties from obtaining access to valuable resources. Furthermore, they contended that allowing exclusive mineral rights would also conflict with existing state laws which provided for public access and use of certain natural resources. In conclusion, they maintained that granting exclusive mineral rights was against established legal principles and should not be allowed under any circumstances.