266 On the Geology of the Key of Sombrero, W. I. 



not sufficient to change the character of the succeeding deposits, 

 but only to establish a separation over small areas. 



The irregular lines of oblique lamination in the upper part of 

 the bed may show that the deposition of sediment was tempo- 

 rarily interrupted over certain small surfaces, e. g., by the fre- 

 quent occurrence of shocks and slight upheavals, or by the acci- 

 dentally unequal distribution of the detritus ; on which account 

 the subjacent material hardened so as not to unite with the 

 detritus subsequently overlaid. 



Bed D. 



§ 16. Character. — Wherever the limits of this bed exist its 

 thickness varies from one to six feet, with an average of four 

 feet; the variations are irregular, as if caused merely by the 

 hollows in the upper surface of the underlying bed. 



The upper two feet of this bed, as a general rule, partake of 

 the character of Lagoon limestone, and its lower portion of Sand 

 limestone, the difference of the two consisting chiefly in the 

 upward-increasing fineness of the grain of the rock and in the 

 better state of preservation of the upper fossils. 



The fossils are small, rarely abundant, and belonging to few 

 species. The first and second modes of fossilization are com- 

 mon, but the fourth prevails towards the bottom of the bed. In 

 the former case the fossils, though colorless, contrast beautifully 

 with the darker tinge of their matrix. Besides these unbroken 

 individuals the rock abounds in minute fragments. 



A strong bedding-line, three-eighths to seven inches deep, 

 averaging half an inch, marks the upper limit. Very fre- 

 quently, too, a separate shallow layer, six to nine inches deep, 

 caps the bed and is marked by a lower bedding-line. This 

 layer is often abruptly broken off and continued by a series of 

 detached fragments of the same thickness and but little out of 

 position. 



Resting upon the upper-bedding-line almost every section 



