Sentence:  Then we obſerve the <WSD>"Frets</WSD>" in the Banks of Rivers that are newly made by any great Land-Flood, which uſually are then very clean, to ſee, if happily we can diſcover any "metalline Stones" in the Sides and Bottoms thereof, together with the Caſt of the Country ("i.e." any earth of a different colour from the reſt of the Bank), which is a great help to direct us, which ſide or hill to ſearch into. 
Meaning of the word marked with <WSD> tags:  The worn sides of riverbanks, where ores or stones containing them accumulate after being washed down from higher ground, which thus indicate to miners the locality of veins of ore.