Sentence:  In some places, however, quartz reefs, payably auriferous while in <WSD>Silurian</WSD> rock, have been followed down to subjacent granite, and have there been found to thin out and become unprofitable ... 
Meaning of the word marked with <WSD> tags:  Of a geologic period within the Paleozoic era; comprises the Llandovery, Wenlock, Ludlow and Pridoli epochs from about 439 to 409 million years ago.