Geoscience Reference
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dissertationis prodromus), upon which most of his reputation is now
based. Here he pulled together information on Tuscan rocks, fossils and
landscape in order to work out the evolution of that landscape. He set
aside the generally accepted assumption that these physical features
had been formed at the Creation, as he comprehended their subsequent
formation. In this topic he made many geological observations that are
now taken for granted by modern geologists: quartz crystals all dis-
played the same angle between faces; the major influence on the for-
mation of topography was running water; like Leonard da Vinci
(1452-1591) who based his conclusions on the nature of fossils on
clear and painstaking observation of specimens found in situ,Steno
recognised that fossils were the remains of once living organisms. He
paid particular attention to the study of sharks' teeth which had until
this time had been considered by many to be petrified tongues, and he
also interpreted and illustrated the sedimentary record. However, he
also made some pronouncements that now seem odd: coal and ash
indicated the presence of former subterranean fires; and volcanoes
occurred through the combustion of carbonaceous material at depth.
Steno explained that the Tuscan landscape had undergone trans-
formation in six stages, which he illustrated in a clever and revealing
diagram (Figure
5.2
). It has been suggested by some commentators that
Fig. 5.2 (cont.)
to the top (youngest): (25) Strata are deposited in horizontal layers. (24)
Erosion of an underground cavern has taken place in the central part of the
diagram. (The strata beneath F and G on the left and right hand sides appear
to be eroded, but this is not the case, and although Steno drew the diagram
in this way, the strata should be taken to extend to the lateral margins of
the diagrams in all cases 20 to 24). (23) The overlying strata have collapsed
into the cavern and produced dipping strata. (22) Subsequently younger
horizontal strata B, A, C (dotted) have been deposited on top of the older
strata, and produced an unconformity. (21) Erosion of another cavern has
occurred, but this time in the younger strata. (20) Further collapse of strata
into this cavern has caused more dipping strata A, and later deposition of
youngest rocks D has occurred. The dotted lines indicate argillaceous or
sandy sequences that may be either unconsolidated or poorly consolidated
while the solid lines represent predominantly lithified horizons. This
diagram has been redrawn from the original (from Winter, Prodromus
(1916), Plate 11).
