352 



NATURE 



[August ii, 1892 



continued down to recent times to gain upon the sea — a result 

 brought about partly by quiet crustal movements, but to a large 

 extent by sedimentation, aided, on the coasts of Florida, by the 

 action of reef-building corals. 



Although volcanic action has long ceased on the American 

 sea-board, we note that in Greenland, as in the West of Scotland 

 and North of Ireland, there is abundant evidence of volcanic 

 activity at so late a period as the Tertiary. It would appear 

 that the great plateau-basalts of those regions, and of Iceland 

 and theFaeroe Islands, were contemporaneous, and possibly 

 connected with an important crustal movement. It has long 

 been suggested that at a very early geological period Europe 

 and North America may have been united. The great thick- 

 ness attained by the Palaeozoic rocks in the eastern areas of the 

 latter implies the existence of a wide land surface from which 

 ancient sediments were derived. That old land must have 

 extended beyond the existing coast-line, but how far we cannot 

 tell. Similarly in North-west Europe, during early Palaeozoic 

 times, the land probably stretched further into the Atlantic than 

 at present. But whether, as some think, an actual land con- 

 nection subsisted between the two continents it is impossible to 

 say. Some such connection was formerly supposed necessary 

 to account for the emigration and immigration of certain marine 

 forms of life which are common to the Palaeozoic strata of both 

 continents, and which, as they were probably denizens of com- 

 pairatively shallow water, could only have crossed from one area 

 to another along a shore-line. It is obvious, indeed, that if the 

 oceanic troughs in those early days were of an abysmal character, 

 a land bridge would be required to explain the geographical 

 distribution of cosmopolitan life-forms. But if it be true that 

 subsidence of the crust has been going on through all geological 

 time, and that the land areas have notwithstanding continued to 

 extend over the continental plateau, then it follows that the 

 oceanic trough must be deeper now than it was in Palaeozoic 

 times. There are, moreover, certain geological facts which seem 

 hardly explicable on the assumption that the seas of past ages 

 attained abysmal depths over any extensive areas. The 

 Palaeozoic strata which enter so largely into the framework of 

 our lands have much the same appearance all the world over, and 

 were accumulated for the most part in comparatively shallow 

 water. A petrographical description of the Palaeozoic me- 

 chanical sediments of Europe would serve almost equally well 

 for those of America, of Asia, or of Australia. Take in con- 

 nection with this the fact that Palaeozoic faunas had a 

 very much wider range than those of Mesozoic and later age?, 

 and were characterized above all by the presence of many cosmo- 

 politan species, and we can hardly resist the conclusion that it 

 was the comparative shallowness of the ancient seas that favoured 

 that wide dispersal of species, and enabled currents to distribute 

 sediments the same in kind over such vast regions. As the 

 oceanic area deepened and contracted, and the land surface 

 increased, marine faunas were gradually restricted in their range, 

 and cosmopolitan marine faunas diminished in numbers, while 

 sediments, gathering in separate regions, became more and more 

 differentiated. For these and other reasons, which need not be 

 entered upon here, I see no necessity for supposing that a 

 Palaeozoic Atlantis connected Europe with North America. 

 The broad ridge upon which the Faeroe Islands and Iceland 

 are founded, seems to pertain as truly to the oceanic depression 

 as the long Dolphin Ridge of the South Atlantic. The trend 

 of the continental plateau in high latitudes is shown, as I think, 

 by the general direction of the coast-lines of North-Western 

 Europe and East Greenland, the continental shelf being sub- 

 merged in those regions for a few hundred fathoms only. How 

 the Icelandic ridge came into existence, and what its age may 

 be, we can only conjecture. It may be a wrinkle as old as the 

 oceanic trough which it traverses, or its origin may date back to 

 a much more recent period. We may conceive it to be an area 

 which has subsided more slowly than the floor of the ocean to 

 the north and south ; or, on the other hand, it may be a belt of 

 positive elevation. Perhaps the latter is the more probable 

 supposition, for it seems very unlikely that crustal disturbances, 

 resulting in axial and regional uplifts, should have been confined 

 to the continental plateau only. Be that as it may, there seems 

 little doubt that land connection did obtain between Greenland 

 and Europe in Cainozoic times, along this Icelandic ridge, for 

 relics of the same Tertiary flora are found in Scotland, the Faeroe 

 Islands, Iceland, and Greenland. The deposits in which these 

 plant-remains occur are associated with great sheets of volcanic 

 rocks which in the Faeroe Islands and Iceland reich a thickness of 



NO. II 89, VOL. 46] 



many thousand feet. Of the same age are the massive basalts 

 of Jan Mayen, Spitzbergen, Franz Joseph Land, and Greenland. 

 These lavas seem seldom to have issued from isolated foci in 

 the manner of modern eruptions, but rather to have welled up 

 along the lines of rectilineal fissures. From the analogy of 

 similar phenomena in other parts of the world it might be 

 inferred that the volcanic action of these northern regions may 

 have been connected with a movement of elevation, and that 

 the Icelandic ridge, if it did not come into existence during the 

 Tertiary period, was at all events greatly upheaved at that time. 

 It would seem most likely, in short, that the volcanic action 

 in question was connected mainly with crustal movements in 

 the oceanic trough. Similar phenomena, as is well known, 

 are met with further south in the trough of the Atlantic. Thus 

 the volcanic Azores rise like Iceland from the surface of abroad 

 ridge which is separated from the continental plateau by wide 

 and deep depressions. And so again, from the back of the great 

 Dolphin Ridge, spring the volcanic islets of St. Paul's, Ascen- 

 sion, and Tristan d'Acunha. 



I have treated of the Icelandic bank at some length for the 

 purpose of showing that its volcanic phenomena do not really 

 form an exception to the rule that such eruptions ceased after 

 Palffiozoic or early Mesozoic times to disturb the Atlantic coast- 

 lines of Europe and North America. As the bank in question 

 extends between Greenland and the British Islands, it was only 

 natural that both those regions should be aff'ected by its move- 

 ments. But its history pertains essentially to that of the 

 Atlantic trough ; and it seems to show how transmeridional 

 movements of the crust, accompanied by vast discharges of 

 igneous rock, may come in time to form land connections 

 between what are now widely separated areas. 



Let us next turn our attention to the coast-lines of the Gulf 

 of Mexico and the Caribbean Sea. These enclosed seas have 

 frequently been compared to the Mediterranean, and the resem- 

 blance IS self-evident. Indeed, it is so close that one may say 

 the Mexican-Caribbean Sea and the Mediterranean are rather 

 homologous than simply analogous. The latter, as we have 

 seen, occupies a primitive depression, and formerly covered a much 

 wider area. It extended at one time over much of Southern Europe 

 and Northern Africa, and appears to have had full communi- 

 cation across Asia Minor with the Indian Ocean, and with the 

 Arctic Ocean athwart the low-lying tracts of North- Western 

 Asia. Similarly, it would seem, the Mexican- Caribbean Sea is 

 the remaining portion of an ancient inland sea which formerly 

 stretched north through the heart of North America to the 

 Arctic Ocean. Like its European parallel, it has been 

 diminished by sedimentation and crustal movements. It re- 

 sembles the latter also in the greatness and irregularity of its 

 depths, and in the evidence which its islands supply of volcanic 

 action as well as of very considerable crustal movements within 

 geological times. Along the whole northern borders of the 

 Gulf of Mexico the coast-lands, like those on the Atlantic sea- 

 board of the Southern States, are composed of Tertiary and 

 recent accumulations, and the same is the case with Yucatan ; 

 while similar young formations are met with on the borders of 

 the Caribbean Sea and the Antilles. The Bahamas arid the 

 Windward Islands mark out for us the margin of the continental 

 plateau, which here falls away abruptly to profound depths. 

 One feels assured that this portion of the plateau has been ridged 

 up to its present level at no distant geological date. But not- 

 withstanding all the evidence of recent extensive crustal move- 

 ments in this region, it is obvious that the Mexican-Caribbean 

 depression, however much it may have been subsequently modi- 

 fied, is of primitive origin.^ 



Before we leave the coast-lands of North America, I would 

 again point out their leading geological features. In a word, 

 then, they are composed for the most part of Archaean and 

 Palaeozoic rocks ; no great linear or axial uplifts marked by 

 much flexure of strata have taken place in those regions since 

 Palaeozoic times ; while igneous action virtually ceased about 

 the close of the Palaeozoic or the commencement of the Mesozoic 

 period. It is not before we reach the shores of the Southern 

 States and the coast-lands of the Mexican-Caribbean Sea that 

 we encounter notable accumulations of Mesozoic, Tertiary, and 

 younger age. These occur in approximately horizontal positions 



I Professor Suess thinks it is probable that the Caribbean Sea and the 

 Mediterranean are portions of one and the same primitive depression which 

 traversed the Atlantic area in early Cretaceous times. He further suggests 

 that it may have been through the gradual widening of this central Medi- 

 terranean that the Atlantic in later times came into existence. 



