Primceml Rivers of Britain. 373 



the fossil state. Take limestones (a various group, comprising chalk, 

 oolite, common limestone, and marble), with the exception of a few 

 that have originated from the chemical arrangement of carbonate of 

 lime, as travertine, all have been made up of organic remains, namely, 

 the calcareous shell or crust, or other support of some of the creatures 

 above mentioned — mostly Shells, Corals, Encrinites, and Foramini- 

 fera ; and only occasional bands of limestone made of fresh- water 

 shells {Paludina, Cyrena, etc.) are found. They tell, however, 

 plainly of their local origin, and are associated with equally powerful 

 and distinct witnesses of the extent and influence of rivers and lakes 

 deltas and lagoons, at many, if not all, periods of the earth's history. 



The chief examples of such evidences as are retained among the 

 known strata, in different parts of the world, will now be noticed in 

 a brief review of the several great groups of geological formations. 



The oldest known strata are the Laurentian, formerly involved in 

 the little understood mass of old schists, slates, and gneiss lying below 

 the Cambrian rocks, but of late years disentangled by Logan and 

 others, and recognized as crumpled beds of masked and altered sand- 

 stones, shales, and limestone, such as constitute any of the later 

 formations. They contain also some thin bands of graphite ; and if 

 this be altered coal, that was accumulated (like the later coal) in salt 

 marshes and lagoons, the existence of land and its concomitant con- 

 ditions is indicated thereby. In other respects the Laurentian strata 

 appear to be of truly marine origin, with shingle and sand-banks of 

 the coasts, and calcareous formations of the deep water. 



The Cambrian rocks have not yet presented evidence of the ex- 

 istence of rivers or lakes. Land, however, there was ; for sand-banks 

 under tidal influence were rippled and sand-cracked, drilled and 

 furrowed by shore-worms and small crustaceans, and pitted by rain- 

 drops. They imbedded here and there the sea- weeds, and, if some 

 decide truly, land plants also (EopTiyton). The Silurian strata are also 

 wholly of marine origin as far as is known ; but in the uppermost 

 formation (the Passage-beds or Ledbury Shales) land plants were 

 brought into the shallows or estuaries, possibly by streams, and 

 remain for us to examine as twigs, branches, spore-cases of 

 Lycopodiaceous plants {Lycopodites, Pachyiheca sphoerica). These 

 Passage-beds lead us into the Old Eed Sandstones of Herefordshire 

 and Scotland; and for these Mr. Godwin- Austen has strongly argued 

 a freshwater origin. That freshwater conditions did predominate 

 when a part at least of the Old Eed of Caithness was formed, the 

 multitudes of Estheria membranacea attest ; and Anodon Jukesii in the 

 Upper Old Eed of Ireland speaks of similar conditions. Wide regions 

 were certainly occupied then as forest-land and by jungle -growth, 

 with Conifers and Cryptogams, whence originated the plant-beds of 

 New Brunswick and the coal of Gaspe. Coniferous trees, too, were 

 not wanting in the Scottish area ; and in either case were probably 

 washed down by rivers to the salt-marshes, with the sand, mud, and 

 vegetable debris that form the associated sandstones and shaly layers. 



On a still larger scale the succeeding period witnessed the action of 

 rain and rivers. These nourished a rich vegetation and swept it 



