Sentence:  Silica and alumina are distinctly the most abundant and characteristic <WSD>petrogenic</WSD> constituents, and with them are most frequently associated those elements toward the extreme <WSD>petrogenic</WSD> end of the periodic table, especially potassium, sodium, and calcium in the order named; and these elements are associated with each other. 
Meaning of the word marked with <WSD> tags:  Of or pertaining to the formation of rock.