THE HISTORY OF GEOLOGY AT OXFORD. 27 



worms ; and of another fossil shell, covered with barnacles, 

 which were adherent to its worn surface, a fact from which 

 he drew several ingenious conclusions. 



Returning to the sediments in which fossils lie buried, 

 Steno pointed out that they are distributed in beds or strata, 

 which are frequently horizontal ; and he concluded that the 

 edges of each bed corresponded originally to the margin of 

 some sea, and its lower surface to the sea-floor. In a series 

 of beds all except the lowest were originally contained be- 

 tween two planes parallel to the horizon. In these state- 

 ments there is contained implicitly the modern doctrine of 

 super-position, i.e. that the order in which beds succeed one 

 another vertically in space is the order in which they have 

 been deposited, or the order of their succession in time. 

 This is explicitly stated by Steno in another paragraph " at 

 what time there was formed any Bed, the matter incumbent 

 on it was all fluid, and by consequence, when the lowest Bed 

 was laid, none of the upper Beds was extant ". 



Steno however pursued his inquiry far beyond this 

 stage ; he proceeded to point out that some strata occupy 

 an inclined or even vertical position, and be rightly inferred 

 that these must have been tilted out of the horizontal by 

 some natural disturbance subsequent to their formation. 



The crowning triumph of Steno's achievement lay in the 

 application which he made of these conclusions to the expla- 

 nation of the structure of Tuscany. 



The structure of Tuscany as Steno conceived it is shown 

 in the following diagram, copied from his work : De Solido 

 intra Solidum Naturaliter Contento. The mountains are 

 for the most part composed of horizontal 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 de- 

 posited as continuous 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 



