i62 Cincinuati Society of Natural History, 



life, they were not the only denizens of that far-away time. 

 The greater part of the limestone is made from the shells and 

 corals, both in their original form, and secondarily as a result 

 of solution. The matter in solution was partly lost, partly 

 deposited in an amorphous form, making a cement to hold the 

 fragments together, and partly in a crystallized form, as cal- 

 cite or dolomite, although the latter is probably a result of a 

 replacement of the calcium molecule by the magnesian. 



The limestone, then, consists of shell and coral fragments 

 of varying size, as.sociated with other forms of fossil life, 

 imbedded in a semi-crystalline matrix composed of still 

 finer fragments, resulting from solution and pressure, firmly 

 cemented together by carbonate of lime, resulting from the 

 solution of shells and corals. 



Looking back into that far-away epoch, one can picture the 

 conditions and changes which aided the formation of these 

 limestones. The molluscan animals and polyzoans inhabited 

 the Ordovician Sea in great numbers, and, dying, sank to the 

 bottom, where they were gradually consolidated under mod- 

 erate pressure and by the action of the water into masses of 

 soft limestone, growing and undergoing changes until the 

 land was elevated and the sea pushed southward. This 

 change was very gradual, for the rocks show no effects of 

 metamorphism or of great disturbance. Many of these shell 

 masses are yet unconsolidated, and are exact counterparts of 

 the coquina, or shell rock, of the P'lorida coast of to-day. 

 Their existence was a quiet one, much like the condition of 

 the Mexican Gulf of the present time ; the changes were few 

 and slow, as if the end had been reached, and henceforth there 

 was to be no change, but underneath flowed the quiet current 

 of progressive evolution, whose work is not visible in a day 

 or year, but ages. And, no doubt, this remnant of the old 

 Silurian Gulf, which we know as the Mexican Gulf, will grow 

 less by new additions much like these old, destined at last to 

 unite completely the two continents, and add a new series to 

 the geological scale of the future. The changes there to-day, 

 slow as they seem to be, are no slower than the changes in the 

 old Silurian Gulf, which geological exploration and study 

 seem to have proven to be nothing more or less than the 

 ancestor o{ the Southern Gulf. 



