Behold this humble passage by Darwin, which is what immediately follows his discussion of the octopus. This passage is a touchstone to several important aspects of what Darwin was doing and thinking, and is a poignant link to what Darwin did not know:
Repost with slight modifications
ST. PAUL’S ROCKS.–In crossing the Atlantic we hove to, during the morning of February 16th, close to the island of St. Paul.
Ah, sorry to interrupt. Saint Paul’s Rocks are in the Atlantic roughly half way between South America and Africa. That geographical information should give you a hint of why these rocks are important!
“Hove to” means “Aye, the fetch is in me maw, she’s dead asea…,” which in turn means “The wind is blowing the wrong way, let’s stop and look at these rocks.”
… continuing …
…The highest point is only fifty feet above the level of the sea, and the entire circumference is under three-quarters of a mile. This small point rises abruptly out of the depths of the ocean. Its mineralogical constitution is not simple; in some parts, the rock is of a cherty, in others, of a felspathic nature; and in the latter case it contains thin veins of serpentine, mingled with calcareous matter. … The circumstance of these rocks not being of volcanic origin is of interest, because, with very few exceptions, the islands situated in the midst of the great oceans are thus constituted.
Darwin understood that a variety of rock types were to be found on the continents, including sedimentary rocks, but the most common sorts of rocks that poked out of the ocean were of volcanic origin, volcanism being a reasonably well understood source of mountain building. It made sense to see volcanoes poking out of the ocean.
Let me also tell you as well that the prevailing theory at that time of the origin of corals, and coral atolls and islands, was that the coral structures grew atop extinct volcanoes. We’ll get back to that important concept later on, as it is central to Darwin’s development as a scientist, and in my view, Darwin’s wrestling with the coral problem is one of the most important things that ever happened in his career and one of the key reasons why everyone knows who Darwin is today.
…. continuing …
As the highest pinnacles of the great mountain ranges probably once existed as islands distant from any continent, we are led to expect that they would frequently consist of volcanic rocks. It becomes, therefore, a curious point to speculate on the changes which many of the present islands would undergo, during the lapse of the countless ages, which would be required to elevate them into snow-clad summits. If we take the case of Ascension, or St. Helena, both of which have long existed in an extinct condition, we may feel assured, before so vast a period could elapse, during the whole of which the surface would be exposed to constant wear and tear, that the mere nucleus or core of the island would remain; perhaps, every fragment of cellular rock having been decomposed, a mass of some compact stone, as phonolite or greenstone, would crown our new Chimborazo.
This passage reveals conceptions of the geological development of the planet: If you see around you high mountains punctuating plains of sedimentary rock, it is easy to have a model of a vast sea or ocean up through which rise mountains, which in turn provide the sediment that eventually displaces the sea. Notice that in this model, the distinction between a continent and an ocean floor is not needed or represented. Also, notice that changes in actual sea level, which certainly happened, are not considered.
There are two things that Darwin, nor any geologist of the time, did not know, one represented in this piece of writing and another only hinted at here but clearly represented elsewhere (and we’ll talk about that later). 1) Plate Tectonics and 2) Eustatic sea level variation during glacial cycles. I’d like to make two strong assertions, and the geology trolls can have at them, but I think I’m right:
1) Darwin made very few errors in interpreting the geology he observed on his voyage, but he did not understand everything. When he did not understand certain phenomena, he did recognize the limitations of his working models, and tended to develop probing questions in preference to untenable explanations. This is because Darwin was a good scientist.
2) If, while Darwin was observing St. Paul’s Rocks, a modern geologist appeared next to him and took ten minutes to explain the very basics of “continental drift,” a very large number of Darwin’s questions about geology would have instead been reformed as contributions to a treatise on plate tectonics. Mantle dynamics (which explain plate tectonics on a deep level!) would have been to his theory of Continental Movement what DNA would later be to his theory of Natural Selection. Additionally, if that same geologist (or a different one … we are so specialized these days) also explained glacial cycles and the movement of water balance between the seas and glaciers, an entirely different set of questions and confusions that are woven into Darwin’s narrative of geology in coastal regions would have become, instead, succinct explanations.
Much about glacial geology was in fact figured out during Darwin’s lifetime, and he expressed (in his autobiography) regret that he had not figured some of this out for himself. But it is a good thing that Darwin did not bump into certain pieces of information about glacial geology, because the concept fascinated him. I suspect that if he had headed in that direction in the decade or so after the voyage of the Beagle, he may have become irretrievably distracted from his work on The Origin.
As one reads The Voyage and other works, but also more broadly among natural historians and geologists of the time (and well into the twentieth century), it is apparent that continental movement is not a factor, but it is also apparent that to a large degree, the vertical movement of sea level is not much of a factor either. Raised beaches will typically be explained as rising land masses. That works, of course, in some cases, but not all. It is almost funny, that sea made of water is more often held constant, while the hard rock geology seen as more plastic. (We’ll see examples of that later on)