Sentence:  The planes in which lie the semimajor and <WSD>semimean</WSD> axes of both the pebbles and quartz-feldspar aggregates coincide with those of the crystalline schistosity (Sj, which follows the bedding (S0). 
Meaning of the word marked with <WSD> tags:  (geometry) Pertaining to half the length of the middle axis on an ellipse (halfway between the major and minor axes).