Prospecting by twig and trench, as shown in one of my favorite books, De Re Metallica by Agricola. Source of image: Wikimedia.
Therefore a miner, since we think he ought to be a good and serious man, should not make use of an enchanted twig, because if he is prudent and skilled in the natural signs, he understands that a forked stick is of no use to him, for as I have said before, there are the natural indications of the veins which he can see for himself without the help of twigs. So if Nature or chance should indicate a locality suitable for mining, the miner should dig his trenches there; if no vein appears he must dig numerous trenches until he discovers an outcrop of a vein.
For a miner must have the greatest skill in his work, that he may know first of all what mountain or hill, what valley or plain, can be prospected most profitably, or what he should leave alone; moreover, he must understand the veins, stringers and seams in the rocks. Then he must be thoroughly familiar with the many and varied species of earths, juices, gems, stones, marbles, rocks, metals, and compounds. He must also have a complete knowledge of the method of making all underground works. Lastly, there are the various systems of assaying substances and of preparing them for smelting; and here again there are many altogether diverse methods. Fibrae—"fibres." See Note 6, p. 70.
 Commissurae saxorum—"rock joints," "seams," or "cracks." Agricola and all of the old authors laid a wholly unwarranted geologic value on these phenomena. See description and footnotes, Book III., pages 43 and 72.
Read De Re Metallica at Project Gutenburg.
Read De Re Metallica at Internet Archive.
This post has been submitted to Accretionary Wedge #43 at In the Company of Plants and Rocks.
UPDATE: Geologic Illustrations Galore -- Accretionary Wedge #43 is now up!