Abstract: The concept of membership cloud was proposed by Li Deyi to overcome the deficiency of Fuzzy theory. It is now developed into a series of theories, including Cloud Module, Cloud Transformation and Cloud Reasoning. Rare application of the Membership Cloud Theory(MCT, briefly) in geosciences has been found so far. Phenomenon of member cloud is common in mineral resources and environmental sciences, such as fuzzy assessment of environmental pollution grade, metallogenetic explaination of geochemical and/or geophysical anomalies, assessment of mining safety, quantitative processing of descriptive geodata, knowledge mining from data warehouse, uncertainty research of location and attribute data, and mineral resource prediction, etc. Reasonable methodologies with suitable mathematical hypothesis are strongly needed to process these problems. The creation of MCT provides edge tools for geo-data processing.