STENO 225 



The structure of Tuscany as Steno conceived it is 

 shown in the diagram on p. 226 (Fig. 81), copied from his 

 work, " De Solido intra Solidum Naturaliter Contento." 

 The mountains are for the most part composed of hori- 

 zontal beds, C, F, but dislocated and sloping towards the 

 low ground, on their flanks. On these fundamental 

 rocks others of a later date, C, A, B, rest discordantly, 

 and thus it appears that Steno was familiar with the 

 important phenomenon now termed an unconformity. 

 Now, since the strata were originally deposited as con- 

 tinuous horizontal layers, it follows that the rocks C, A, B 

 at one time extended in an unbroken sheet between the 

 flanks of the surrounding mountains, as shown in Fig. 2. 

 Further, since the bed B, A, C could not have been self- 

 supporting, but must have rested on older sediments, the 

 gap below it must have been filled up with a succession 

 of horizontal deposits as shown in Fig. 3. This state of 

 things, again, was not original ; there was a time when 

 these sediments did not exist, and the Tuscan district 

 appeared as in Fig. 4. Once more, the argument which 

 applied to the latest rocks holds equally for the older ones 

 G, F, so that we must restore these, first as represented 

 in Fig. 5, and next as in Fig. 6. Beyond the last stage, 

 when Tuscany lay inchoate, a mass of sediments beneath 

 a primaeval sea, we can only take one step farther back 

 into the abyss of time, and this brings us to a period 

 antecedent to the deposition of any sediment whatever, 

 and to a state of Tuscany which Steno does not venture 

 to represent by a diagram. 



Steno did not regard the explanation, just set forth, 

 as restricted in its application to Tuscany ; but distinctly 

 affirmed, from his knowledge of other countries, that 

 similar reasoning would hold for every part of the world 

 where stratified rocks occur. 



The first to logically demonstrate the true nature of 



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