It is scarcely necessary to draw the obvious conclu sion from these statements. In no case can a naturalist decide exclusively on the genus or character of any fossil fish, so as to know whether it has been an inhabitant of fresh or salt water ; and if the habits of this class of animals are as versatile as we have attempted to prove, the difficulty becomes insurmountable. Even the cotace ous fish might be found in the earth, without its thence following that they belonged to a marine deposit ; if the Delphinus leueas, as is probable, is now buried in the inundations of gravel which attest the inundations of the Hudson.
It is easy to understand the value of these facts in rea soning about many deposits of the remains of aquatic ani mals. With respect to the Paris strata, it does not, how ever, apply in such a manner as to invalidate the evidence of two alternations of terrestrial and marine depositions. These are put out of all question by the presence of the remains of land animals and vegetables in both. It has been suggested that, in this case, there have been inroads and retreats of the sea, alternating with fresh water lakes. As far as we can discern, it does appear that salt water and fresh have alternately covered the same land; but it is necessary to be more particular in stating the circum stances under which these changes have taken place.
The simplest solution is, that the last marine deposit had been produced while the cavity in question had formed a bay of the ocean. The gradual filling up of this bay by the usual alluvia of rivers may have made it a plain ; or, under circumstances of which examples are not want ing in our own time, may have converted it into a fresh water lake, and ultimately into a plain ; thus enabling it to receive and preserve the remains of fresh aquatic animals, and of the inhabitants of dry land. This progress may now be traced in many of the Scottish lochs, and thus the first step, or the first succession of terrestrial to marine deposits, is easy.
Beyond this, the same theory serves our purpose no longer, since the land has now been raised above the level of the ocean. It is necessary to depress it once more beneath that, before the third deposit, consisting of marine remains, can be formed. It is evident that, whe ther these remains are admitted to be marine or not, the depression of the land is almost equally necessary ; unless the formation of a new fresh water-lake, at a higher level, can be admitted to have occurred, from changes of the elevation of the soil at the exit of the draining waters. But there is no reason to be startled at a depression of land adequate to the admission of the ocean. The case is
precisely analogous to that which has in so many places happened in the instance of the coal strata, and which, in all these, appears to have been of partial occurrence.
That the earth has undergone various changes of level, more or less general, is demonstrated by innumerable ap pearances; it is even probable that they have occurred in many well known instances, in times very recent ; that they occur every clay in the case of volcanic actions, in the subsidence of mountains, or the depression of plains from earthquakes, and in the formation of new islands, or the elevation of the coral plains of the Pacific Ocean. If these changes of level be admitted, the alternation of a second fresh-water deposit is no more difficult than that of a first, and the same reasoning applies to the last stage of both. Geology presents numerous facts, not less re fractory. To those difficulties, which have been long known, we become reconciled by habit; while we are startled by the appearance of novelty, where, in truth, there is nothing new in principle.
Admitting, then, repeated changes of the earth's sur face, as we cannot refuse to do unless we voluntarily shut our eyes to exclude the light, we find a general chain of analogy linking all these difficulties together; and it re quires but a slight degree of ingenuity, in most cases, to supply the various accessary circumstances which at what ever place and at whatever time, have modified the general results, so as to have produced the varieties that occur in our investigations.
It has been objected to the theory of alternating saline and fresh deposits, that, in the basin of Paris, the rocks arc often the same in both. It is evident that this objec tion is founded on the hypothesis that all the materials of rocks have been in solution in water; and, on a further hypothesis, that a saline menstruum could not have dis solved the same materials as a fresh one. The minor hypothesis requires no answer; and it is a sufficient answer to the principal one, that there is not a shadow of evi dence to prove that the materials of rocks ever have, or ever could have been, in solution in water. On the con trary, those of the stratified substances have been almost entirely deposited from a state of suspension ; and it is perfectly easy to conceive, that, however the waters of the receptacle might have changed, the deposited materials must have been identical, or have varied, where they did vary, independently of these changes.