On the appearance of land, or during the transition of the earth from its chao tic to its habitable state, rocks, which, from this circumstance, are denominated transition rocks, were formed. In these rocks the first slight traces of petrifac tions, and of mechanical depositions, are to be found. The species of rocks Which come under this class are, the transition Time-stone, transition-trap, gray-wacke, and flinty slate. The petrifactions are, corallites, encrinites, pentacrinites, en trochites, and trochites. The lime-stone of Derbyshire is said to be of this kind. As the former class of rocks were purely of chemical formation, so the contents of these are chiefly chemical productions, mingled with a small proportion of me chanical depositions. To explain the cause of this mixture we are referred to the pe riod of their formation, that at which the summits of the primitive mountains just appeared above the waters, when, by the attrition excited by the motion of the waves, and which we are reminded ex tends to no great depth, particles of the original mountains were worn off and de posited.
As the height of the level of the ocean diminished, so would the surface on which its waves acted increase, and of course the quantity of the mechanical deposi tions. Hence these are much more abun dant in the rocks of the next formation, which are denominated flcetz rocks, on account of their being generally disposed in horizontal or flat strata. In these, pe trifactions are very abundantly found, having been formed whilst vegetables and animals existed in great numbers. These rocks are generally of very wide extent, and commonly placed at the feet of pri mitive mountains. They are seldom of very great height, from whence it may be inferred, that the water had considerably subsided at the time of their formation, and did not then cover the whole face of the earth. Countries composed of these rocks are not so rugged in their appear ance, nor so marked by rapid inequalities, as those in which the primitive and tran sition rocks prevail. The formations of this class are supposed to be, 1, first or old red sand-stone ; 2, first or oldest flcetz lime-stone ; 3, first or oldest flcetz gyp sum ; 4, second or variegated sand-stone ; 5, second flcetz gypsum ; 6, second flcetz or shell lime-stone ; 7, third flcetz sand stone ; 8, rock-salt formation ; 9, chalk formation ; 10, flcetz-trap formation ; 11, independent coal formation ; 12, newest flcetz-trap formation.
Most of the rocks which have been just enumerated are covered by a great for mation, which is named the newest fhictz trap. This formation also covers many of the high primitive mountains : it has but little continuity, but is very widely distributed. It contains considerable
quantities of mechanical deposits, such as clay, sand, and gravel. The remains both of vegetables and animals also occur very abundantly in these deposits. heaps of trees and of parts of plants, and an abundance of shells and other marine pro ductions, with thehorns of stags, and great beds of bituminous fossils, point out the lateness of the period when this formation was deposited. In this formation several rocks occur, which are also met with in other flcetz formations; but the following are supposed to be peculiar to this class, basalt, wacke, gray-stone, porphyry, slate, and trap tuff. These rocks are said to have been formed during the settling of the water consequent to a vast deluge, which is supposed to have taken place when the surface of the earth was cover ed with animals and vegetables, and when much dry land existed. From various ap pearances observed in these rocks it is concluded, that the waters in which they were formed had risen with great rapidi ty, and had afterwards settled into a state of considerable calmness.
The collections and deposits derived from the materials of pre-existing masses, worn do*n by the powerful agency of air and water, and afterwards deposited on the land or on the sea coasts, are termed alluvial, and are, of course, of much later formation than any of the preceding classes. These deposits may be divided into, I. Those which are formed in moun tainous countries, and are found in vallies, being composed of rolled masses, gravel, sand, and sometimes loam, fragments of ores, and different kinds of precious stones. 2. Those which occur in low and flat countries, being peat, sand, loam, bog iron oar, nagelflech, calc-tuff; and calc• sinter : the three latter being better known by the names breccia, tufa, and stalactite.
In this ingenious system, in which so much knowledge of the subject prevails, and in which the marks of long and pa tient investigation are evident, a very close accordance with geological facts is generally observable. Some few difficul ties however occur, particularly it seems with respect to the new trap formation ; since, although the appearances which this is intended to explain do not better agree with any other supposition, still the rising of the waters, whilst they yet covered the summits of primitive moun tains, has much the appearance of a sup position made up for this particular pur pose ; and as, at the same time, it appears to be warranted by no other phenomena, it seems to require some further consi deration, before it is fully admitted.
For more particular observations on the various characters, and on the differ ent classes of rocks, see Rams.