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Show 568 CORAL FOll.MA1'10NS. April, 1836. of elevation, but that we possessed no means of knowing it. I conceive it is by eliminating this source of doubt, that the alternate bands of opposite movement, deduced from the configuration of the reefs, directly bear on this law. I need not do more than simply state, that we thus obtain (if the view is correct) a means of forming some judgment of the prevailing movements, during the formation of even the oldest series, where volcanic rocks occur interstratified with sedimentary deposits. Any thing which throws light on the movements of the ground is well worthy of consideration ; and the history of coral reefs may, in another manner, elucidate such changes in the older formations. As there is every reason to believe that the lamelliform corals grow only abundantly at a small depth, we may feel sure, where a great thickness of coral limestone occurs, that the reefs on which the zoophytes flourished, must have been sinking. Until we are enabled to judge by some means what were the prevailing mo~ements at different epochs, it will scarcely ever be possible to speculate with any safety on the circumstances under which tlie complicated European formations, composed of such different materials and in such different states, were accumulated. Nor can I quite pass over the probability of the above views illustrating those admirable laws first brought forw~rd by Mr. Lyell,-of the geographical distribution of plants and animals, as consequent on geological changes. M. Lesson has remarked on the singular uniformity* of the lndio-Polynesian Flora throughout the immense area of the Pacific ;the dispersion of forms having been directed against the course of the trade-wind. If we believe that lagoon islands, those monuments raised by infinite numbers of minute architects, record the former existence of an archipelago or continent in the central :part of Polynesia, whence the "' Perhaps this is stated rather too strongly ; but M. Lesson, of course, had grounds for his assertion. April, 18.36. CORAL FORMATIONS. 569 germs could be disseminated, the problem is rendered far more intelligible. Again, if the theory should hereafter be so far established, as to allow us to pronounce that certain districts fall within areas either of elevation or subsidence, it will directly bear upon that most mysterious question,whether the series of organized beings peculiar to some isolated points, are the last remnants of a former population, or the first creatures of a new one springing into existence. Briefly to recapitulate. In the first place, reefs are formed around islands, or on the coast of the mainland, at that limited depth at which the efficient classes of zoophytes can live; and where the sea is shallow, irregular patches may likewise be produced. Mterwards from the effects of a series of small subsidences, encircling reefs, grand barriers, or lagoon islands, are mere modifications of one necessary result. Secondly, it can be shown on the above views, that the intertropical ocean, throughout more than a hemisphere, may be divided into linear and parallel bands, of which the alternate ones have undergone, within a recent period, the opposite movements of elevation and subsidence. Thirdly, that the points of eruption seem invariably to fall within areas subject to a propulsion from below. The traveller who is an eyewitness of some great and overwhelming earthquake, at one moment of time loses all former associations of the land being the type of solidity, so will the geologist, if he believe in these oscillations of level (the deeply-seated origin of which is betrayed by their forms and vast dimensions), perhaps be more deeply impressed with the never-ceasing mutability of the crust of this our World. |