lxxiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY;, | May I9II,. 



necessarily confined to dealing with statical conditions. We may 

 make use of our knowledge of the progression of events likely to 

 follow during the normal geographical development of the area 

 of our study, to institute comparisons with the events recorded in 

 a continuous succession of stratified rocks. For example, I have 

 personally found my ideas of Carboniferous conditions clarified 

 after searching for like conditions or succession of conditions in the 

 present. 



The Gulf of Mexico, with the Mississippi delta, the island of 

 Cuba, and the Caribbean Sea beyond, maybe used to focus our ideas 

 on a possible succession of events not unlike the progress of affairs 

 during the Carboniferous Period. At the present time the region 

 may be said to be passing through a Carboniferous Limestone phase. 

 We have (a) a great^ northern river with its delta, (b) a central 

 sea, (c) an island to compare with the lowering ' Mercian High- 

 lands,' and (d) the great ' Culm ' depths of the Caribbean Sea.. 

 Conditions at present favour the deposit of organic material over 

 vast areas in the region, the chief sediment coming in from the 

 Mississippi and building there a delta under conditions very similar 

 to those that prevailed when the Lower Carboniferous rocks of the 

 Scottish and Northern Pennine areas were being laid down. 



Supposing that the geography of the region were to remain as 

 at present, or if events were to be hastened by a general uplift of 

 the area, we should have a progression of phenomena not altogether 

 unlike those recorded in the Carboniferous strata of Britain, that 

 is, the gradual pushing out of the deltaic Coal-Measure conditions 

 from the northern continent and from the ' Mercian ' island. 

 Everywhere the deltaic conditions will be preceded by a marine 

 sediment of the type of the Millstone Grit ; and, where conditions 

 favour it, the actual Coal-Measure conditions may be locally fore- 

 shadowed in this deposit. The ' Millstone Grit' type of formation 

 will be homotaxial throughout, it will represent the persistence of 

 a condition, it will rest everywhere upon organic deposits, and will 

 in turn be succeeded by deltaic Coal Measures. But neither it nor 

 any member of the Coal-Measure sequence, except perhaps the 

 coal-seams themselves, will be, as a whole, a true time-unit ; in 

 some places it will be of the same age as part of the organic deposits, 

 in other places it will be contemporaneous with the formation of 

 parts of the Coal Measures. 



Another example may be drawn from the known progression 

 from the Ordovician to the Old Eed Sandstone. The British 

 Ordovician geography finds its nearest modern parallel in the 



