5 16 THE JOURNAL OF GEOLOGY. 



zose beach sands. Like the tufa, it is independent of the material 

 upon which it gathers, but the possibility of a local supply of 

 lime exists in the discharge of surface waters below low tide. 

 Under the microscope the material shows a dense, fine-grained 

 groundmass of lime with admixture of fine clay, including grains 

 of quartz and cavities filled with coarsely crystalline calcite. 



A case, which is probably more typical of what may occur 

 now, or may have occurred in past ages at the mouths of rivers 

 and in shallow seas, is that of the limestone deposited beyond the 

 delta of the Rhone. This is referred to by Thoulet, 1 and is 

 described by Lyell, 2 who says: "In the museum at Montpelier 

 is a cannon taken up from the sea near the mouth of the river, 

 imbedded in crystalline calcareous rock. Large masses also are 

 continually taken up of an arenaceous rock, cemented by calcare- 

 ous matter, including multitudes of broken shells of recent 

 species." Lyell attributes the precipitation of lime to evapora- 

 tion of the Rhone water, which, when it is spread upon the salt 

 water, he compares to a lake. But this one cause is no doubt 

 combined with the chemical and mechanical conditions which 

 have been suggested in the preceding discussion. These con- 

 ditions are favored at the mouth of the Rhone by the salinity of 

 the Mediterranean and the absence of strong currents. 



The examination of a few thin sections of limestone of dif- 

 ferent ages, from Cambrian to the present, shows that they have 

 three principal types of structure. There are those which re- 

 semble the Everglades limestone in that they consist of more or 

 less coarsely crystalline calcite, yet include unaltered organic 

 remains. Of these the Trenton limestone and the marbles of 

 corresponding age in Tennessee, which occur interstratified with 

 unaltered calcareous shales, are the most striking examples 

 examined. Cambrian limestones and the Knox dolomite show 

 similar crystalline structure. The second type, the precipitated 

 sediment which forms the muds of the Everglades and which 

 was deposited in Lake Bonneville is represented by specimens 



1 Op. cit, p. 270. 



2 Principles of Geology, Vol. I, p. 426. 



