Sentence:  <WSD>Peach</WSD>, which is a word used by the Cornish miners, in a generic sense, to denote all minerals of the chloritic family—and is consequently a very convenient word—seems to be essentially the "mother" of tin; but the experience of Cornwall goes to show that <WSD>peach</WSD> alone does not produce a permanent tin mine: an intermixture of quartz is necessary to give what miners call "strength" to the lode. 
Meaning of the word marked with <WSD> tags:  A particular (rock) found in (tin) mines, sometimes associated with (chlorite).