NICOLAUS STENO 173 



If strata are no longer in a horizontal position, it indicates, says 

 Steno, subsequent disturbance of them ; and this may be due either 

 to uplift "by violent thrusting up of the strata," or "spontaneous 

 slipping or downfall of the upper strata after they have begun to 

 form cracks, in consequence of the withdrawal of the underlying 

 substance, or foundation " (p. 231). 



These changes in position of the strata are according to Steno 

 the chief cause of mountains, and he pretty clearly distinguishes 

 three of the more important mountain types; namely, (i) block or 

 fault mountains, (2) volcanic mountains, and (3) mountains of ero- 

 sion. The relation of earthquakes to the formation of mountains is 

 indicated with a much nearer approach to present beliefs than is to 

 be found in any save Robert Hooke and comparatively recent 

 writers. 



The fissures which form in the strata were recognized by Steno to 

 be the passageways or channels for the movement of underground 

 water, and for subterranean gases as well. These crevices are thus 

 the places where veins of mineral are formed. The storehouses of 

 the precious metals being brought about by natural processes, the 

 foolishness of those who employ the divining rod for the locating of 

 them is pointed out. An imperfect notion of the manner of replace- 

 ment of one mineral by another seems to have been gained by Steno 

 from his studies. 



In the description of the figures ^ — a most important part of the 

 essay — ,a. clear conception is revealed of the relative order of age of 

 strata, of the alternation of transgression and recession of the sea 

 over the same places, and of the nature of a structural unconformity, 

 whereby one set of strata comes to overlie another from which it 

 differs in its lesser angle of inclination. Here Steno gives us the 

 results of his careful field observations in the vicinity of Florence. 

 His figures may, therefore, be regarded as^ the earliest geological 

 sections ever prepared. 



Over the origin of fossils war had long been waged in Steno's 

 time. Like Leonardo, a century and a half before, Steno declared 

 that fossils were petrified remains of plants and animals which had 

 once existed. 



Steno's activity in biological studies is brought out in his elaborate 

 examination of the structure of the shells of mollusks. His descrip- 

 tion of the subdivisions of the shells and the division of these into 



