Sentence:  The Na-K-Ca <WSD>geothermometer</WSD> and silica <WSD>geothermometer</WSD> are discussed as tools for locating and evaluating the areas prospectively valuable for geothermal development. 
Meaning of the word marked with <WSD> tags:  A proxy that allows for geothermometry, the determination of the temperature of a geological material from the past or existing at great depth.