252 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



By the expression " external relations" is meant such as do not 

 directly lead us to the consideration of structural or physiological 

 distinctions ; the latter belong to a separate and very important 

 field of investigation. 



§ 1. Conditions and characters of Sediments. — It being a gene- 

 rally received fact that palaeozoic fossils usually passed their lives 

 in and about the sedimentary strata in which they are found, some 

 very brief remarks on the origin and nature of the latter may be 

 excused. The agents concerned in the formation of strata may be 

 divided into two great classes : — 1st. The constant and superficial, 

 often called " Neptunian." 2ndly. The modifying, or occasional 

 and subterranean, often called "Plutonic." 



The first, or constant, causes are mechanical and chemical, both 

 being in universal and ceaseless action. The mechanical agencies 

 operate, on land, by comminution and degradation, and in the 

 ocean, by removing and redepositing suspended matters by the 

 means of currents, pelagic, estuarine, and fluviatile — all liable to 

 variations, stoppage, and reversion. Their greatest activity is 

 experienced, as is well known, near the land, where broken up and 

 removable substances are most plentiful. The nature of the sea- 

 bottom is therefore usually determined by the geological constitution 

 of the neighbouring land and its coasts, the very outline of the 

 latter having a powerful influence, as they are high or low, straight 

 or curved. Of the details of these currents in the palaeozoic times, 

 their force, direction, (fee, we know but little as yet ; for they w^ere 

 not those of the present day — the position, form, and quantity of 

 the dry land being different. Many appearances exist, however, 

 tending to prove that during the deposition of the middle stage of 

 the Silurian system of New York a broad current set in from the 

 region now sunk under the Atlantic, in a W.N.W. direction, and 

 overspread that State and the countries still more westerly with 

 various forms of detritus, which at first (on the eastern edge of 

 middle North America) was a conglomerate, and then a grit, and 

 successively a deposit arenaceous, argillaceous, and calcareous, as it 

 progressed into the deep western seas of the epoch. Precisely 

 similar phenomena occur in Bohemia, except that the currents and 

 their contents come from opposite quarters, N.E. and S.W., at dif- 

 ferent stages of the Silurian era*. 



The chemical causes are everywhere in action, dissolving and con- 

 verting, simultaneously with the mechanical. In the high seas they 

 are often alone. There is reason to believe that the mineral ingre- 

 dients of sea- water are always undergoing minute changes of com- 

 position, which result in precipitations, — ^by far the most part of this 

 being effected by the animals inhabiting it, and principally by the 

 E-hizopods, assisted by the minute Diatomaceous vegetables. 



The second class of causes, the occasional and modifying, employed 

 in the deposition or the derangement of strata, consist of insensible 

 oscillations of level, crust-ruptures, or elevations. The slow de- 

 pressions and upheavals of the earth's surface, which have been 

 * Barrande, ' Systeme Silurienne Boheme,' p. 62. 



