“At a time when fantastic metaphysics 
were rife, Steno trusted only to in- 
duction based upon experiment and 
observation.” But the Prodromus re- 
ports no real experiments and only 
a modest number of observations. It 
succeeded primarily because Steno 
followed a metaphysic congenial with 
our own, but relatively new in his time. 
Geologists have also judged Steno 
inappropriately by searching the text 
for gems of “modern” insight, rather 
than by understanding its argument 
as a totality. Thus, the commonest 
statement about the Prodromus, often 
the only statement made by geologists, 
holds that Steno presented the crys- 
tallographic law of the constancy of 
interfacial angles — that however 
much the size and shape of crystal 
faces vary, the angles between them 
remain the same. Well, perhaps he 
did, but the “law” appears as two 
throwaway lines in a figure caption, 
and has little relation to Steno’s major 
theme or argument. (It arises simply 
as a corollary to his speculations about 
inferring the motion of fluids from 
the form of solids precipitated from 
them.) 
No, the Prodromus is, as its title 
states, about solids in solids and their 
proper classification by mode of ori- 
gin. It is founded upon two great taxo- 
nomic insights: first, the basic rec- 
ognition of solids within solids as a 
coherent category for study and, sec- 
ond, the establishment of subdivisions 
to arrange solids within solids accord- 
ing to the causes that fashioned them. 
Steno uses two criteria for his sub- 
divisions. (They are blessedly obvious 
once you state the problem, but 
Steno’s revolution is the statement it- 
self.) First, in what might be called 
the principle of molding, Steno argues 
that when one solid lies within another, 
we can tell which hardened first by 
noting the impress of one object upon 
the other. Thus, fossil shells were solid 
before the strata that entomb them 
because shells press their form into 
surrounding sediments just as we 
make footprints in wet sand. But sur- 
rounding rocks were solid before the 
calcite veins that run through them 
because the calcite fills preexisting 
channelways just as jello matches the 
flutes of a mold. The principle of mold- 
ing allows us to establish the temporal 
order of formation for two objects in 
contact. In a world still regarded by 
many of Steno’s contemporaries as 
formed all at once by divine fiat, this 
criterion of history struck a jarring 
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