August 31, ISSS.] 



SCIENCE. 



•29:> 



oftentimes these schists are disposed lilce tlie lavas 

 ejected from one series of volcanic vents. Snppose, 

 for example, that Etna or Vesuvius should become 

 extinct. In the course of ages the rains would 

 obliterate the craters, and reduce the lavas to a 

 rounded dome of greater or less regularity. We 

 should recognize the volcanic origin of the mountain 

 in the absence of craters from the lithological simi- 

 larity of the rocks to tho<e known to have been 

 melted and ejected from vents, while the disposal of 

 the material in a conical attitude shows us that it 

 might once have been covered by craters. So we find 

 in our eastern country many domes of diabasic or 

 protogenic schists, whose volcanic origin may be 

 predicated, both from their lithological character and 

 physical aspect. 



Now, this volcanic group of Huronian times in- 

 dicates the existence of a greater degree of igneous 

 activity than has been described for the paleozoic 

 ages, even those of Great Britain ; and consequently 

 this is an indication pointing significantly towards 

 the predominance of thermal influences in the still 

 earlier periods. In tlic Laurentian age the fiies 

 should have been yet more vigorous, because the 

 time of universal igneous fluidity was less remote. 



2. A careful study of the crystalline rocks of the 

 Atlantic slope indicates the presence of scattered 

 ovoidal areas of Laurentian gneisses. Those best 

 known have been described in the Geology of New 

 Hampshire. Instead of a few large ."yncliiial troughs 

 filled to great depths with sediments, the oldest 

 group is disposed in no less than twenty-two areas of 

 small size, scattered like the islands in an archipelago. 

 In a chapter upon the physical history of the state, I 

 have proposed the theory that the earlie>t land with- 

 in its limits consisted of this series of islands, not 

 packed as closely together as now, in an an'a of per- 

 haps three thousand five hundred square miles, but 

 as ifiucb move widely separated as would be deter- 

 mined by smoothing out the various anticlinals and 

 synclinals that were formed later. By reference to 

 our maps in Maine, Massachusetts, New Jersey, Penn- 

 sylvania, Virginia, North Carolina, and Georgia, many 

 similar ovoidal Laurentian areas may be specified, 

 usually larger than those of New Hampshire. This 

 may be due partly to a kss thorough knowledge of 

 the exact areas occupied by this older gneiss, .ind 

 partly to the existence of a greater number of vol- 

 canic vents, giving rise to a more widely spread and 

 thicker mass of ejected material. Over the Atlantic 

 slope and Canadian highlands these primeval islands 

 have, in later periods, been cemented together by a 

 subsequent deposition of material; but in Missouri, 

 Arkansas, and Texas, we recognize, even now, these 

 early islands. 



3. The lithology of the Laurentian and other crys- 

 talline rocks is very like that of igneous ejections. 

 It is proper at this point to recall the proper restric- 

 tion of the term Laurentian. As originally defined 

 by Lofan, it included every formation tliat ante- 

 dated the Huronian. In the Report upon the geology 

 of Canada for 1877-78, Selwyn proposes to restrict 

 the Laurentian outcrops to "all those clearly lower. 



unconformable, granitoid, or syenitic gneisses in 

 which we never find interstratificd bands of calca- 

 reous, argillaceous, arenaceous, and conglomeratic 

 rocks." The Hastings and Grenville series, and all 

 the schists containing the eozoon, are excluded from 

 the Laurentian by this definition, as well as the 

 Bethlehem, Lake Winnipiseogee, and Montalban 

 groups of the Atlantic slope. The Laurentian is 

 azoic, the other groups eozoic; and, unless newer 

 distinctions are to be made hereafter, it looks as if we 

 might claim these various azoic Laurentian islands 

 as the first-formed dry land, as they certainly are the 

 nuclei of the existing continents. 



There are no minerals in these Laurentian islands 

 that do not occur in eruptive granite; and the schis- 

 tose structure is often so faint that the field geologist 

 need not be blamed if he acknowledges his inability 

 to detect it. Likewise we discover the same fluidat 

 inclusions and the vacuoles that pertain to granite. 

 If we follow Sorby and Clifton Ward in saying that 

 gr.anite has been formed beneath a pressure equiva- 

 lent to a weight of forty thousand feet of strata, the 

 same must be said of the early gneisses. With this 

 general assertion of the identity of gneiss and erup- 

 tive granite, we must be satisfied at present, without 

 entering into detail. 



4. The analogy of the origin of oceanic islands 

 at the present day suggests the igneous derivation 

 of the Laurentian areas. Most of the high islands of 

 the Pacific are composed of lava, with the volcanoes 

 frequently in action. Hawaii, of the Hawaiian 

 group, may illustrate their position and shape. Its 

 area above the water-line is 4,210 D miles, and its 

 cubical contents above the sea-level are about the 

 same with those of New Hampshire. It rises from a 

 plateau over 1(!,000 feet deep, thus forming a cone 

 ;50,000 feet high, whose cubical contents must be 

 twenty times greater than the portion making dry 

 land. The length of the entire series of islands, all 

 of similar character, is 350 miles, and the area of the 

 l).a«e of the lava must be about 1(10,000 D miles. 

 These cones have been built up by the accumulation 

 of lava ejected from the interior of the earth, and they 

 are entirely isolated, the nearest land being 1,000 

 miles distant. The ground-plan of this volcanic 

 ni.iss is that of two elliptical areas, either of which is 

 like some of our Laurentian islands, and is certainly 

 as large as any of these ancient lands south of the St. 

 Lawrence. The land area of the Hawaiian Islands is 

 less than th.at of Massachusetts, but their base must 

 he equal to the whole of New England and New 

 York combined. Surely it cannot be avowed that 

 volcanic areas are too small to be compared with the 

 sp.ice occupied by our oldest formation. 



The so-called lowlands are likewise of volcanic ori- 

 gin; since coral polyps have built up reefs upon the 

 igneous area after the disappearance of the fire, and 

 the Hawaiian areas are encircled by reefs. After the 

 volcanoes have become coM, loo-^e material would be 

 worked in between them, coral reefs would grow, 

 and, in various ways, the land area would be enlarged, 

 and finally an archipelago may become a large island. 

 It needs only time and a repetition of these construe- 



