Sentence:  Members of higher and lower social status often cluster around this nucleus , so that Protestant figures on social class give the impression of spread over all social classes ; but this is deceptive , for the core of membership is concentrated in a single social and economic <WSD>stratum</WSD> . 
Meaning of the word marked with <WSD> tags:  one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)