172 FOREWORD 



the action of the larger crystal, but existed first and were enveloped 

 through continued growth of the crystal nucleus. 



In the realm of geology we owe to Steno the first clear enuncia- 

 tion of some of those great principles which to-day we assume to be 

 axiomatic only because so much has been built upon them as a 

 foundation. That rocks in the main result from sedimentation in 

 water is thus expressed in the Prodromus (p. 219): 



" The strata of the earth, as regards the manner and place of pro- 

 duction, agree with those strata which turbid water deposits." 



The reasons for this belief are most cogent : " The strata of the 

 earth are due to the deposits of a fluid, (i) because the comminuted 

 matter of the strata could not have been reduced to that form 

 unless, having been mixed with some fluid and then falling from its 

 own weight, it had been spread out by the movement of the same 

 superincumbent fluid; (2) because the larger bodies contained in 

 these same strata obey, for the most part, the laws of gravity, not 

 only with respect to the position of any substance by itself, but also 

 with respect to the relative position of different bodies to each 

 other" (p. 227). 



It is further clearly shown how marine deposits may be distin- 

 guished by their character from those deposits which are laid down 

 in fresh water upon the continents, as well as from the ejectamenta 

 of volcanoes. The origin of variation in the character of strata from 

 place to place, and of the alternation of layers of different characters, 

 are all discussed with a clear understanding of the actual conditions. 

 The great principle that the order of superposition of beds deter- 

 mines the age of formations, is given its first expression (p. 230) :. 



" At the time when any given stratum was being formed, all 'the 

 matter resting upon it was fluid, and, therefore, at the time the 

 lowest stratum was being formed, none of the upper strata existed." 



Likewise it is pointed out that sedimentary formations were either 

 laid down in definite basins of deposition or were universal in their 

 extent. The original horizontality of sedimentary formations is now 

 regarded as one of the great fundamental principles of geology. 

 Steno says of the strata " that the upper surface was parallel to 

 the horizon, so far as possible ; and that all strata, therefore, except the 

 lowest, were bounded by two planes parallel to the horizon. Hence 

 it follows that strata either perpendicular to the horizon or inclined 

 toward it, were at another time parallel to the horizon " (p. 230). 



