258 On the Geology of the Key of Sombrero, W. J. 



the same vertical lines at different heights on the sides of quar- 

 ries. In all these determinations the specimens were coarsely 

 pulverized, and distilled water at 80° F. employed. 



E 



III. IV. 



2.64 2.62 



2.61 

 2.49 



2.52 2.43 



c l 



B l 

 f 



I 



The upward increase of the density of the material of each 

 bed is thus displayed, except in bed A, in which Calc Spar occurs 

 in the fossils. 



§ 8. Modes of Fossilization. — In these beds there are five 

 modes by which the fossil organism is represented. 



First, The original shell (or its thickest portion) remains, 

 retaining all its physical and chemical characteristics. JSTo 

 corals are thus preserved. 



Secondly, The organic matter has decomposed and been 

 removed, but the inorganic remains as a smooth white mass, 

 retaining the form and markings of the original shell, very soft 

 to the touch and friable. Specimens of Porites are also fre- 

 quently thus preserved. 



Thirdly, In corals and the larger shells, the cavity between 

 the casts is partly empty, its sides being more or less lined with 

 numbers of small flattened spherules possessing the appearance 

 and variation in size of shot. According to their arrangement 

 forms are produced resembling coats-of-mail, strings of beads, 

 and interlacing twigs. They have probably been caused by 

 the partial redeposition of the carbonate of lime, from the solu- 

 tion of the white filling mentioned under the second mode. 



