Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/342

Rh 328 and serpentine. But perhaps their most intelligible sections are those which they present in Bavaria and Bohemia between the valley of the Danube and the hea(l- waters of the Elbe. They are there divided into two well-marked groups—(a) red gneiss, covered by (b) grey gneiss. According to Giimbel the former (called by him the Bojan gneiss) may be traced as a distinct formation associated with granite, but with very few other kinds of crystalline or schistose rocks, while the latter (termed the Hercynian gneiss) consists of gneiss with abundant interstratification of many other schistose rocks, graphitic limestone, and serpentine. The Hercynian gneiss is overlaid by mica-schist, above which comes a vast mass of argillaceous schists and shales. Giimbel some years ago found in the marbles associated with the younger gneiss what he considered to be an organism of the same genus as the Eozoon of Canada, to which reference will immediately be made. He named it Eozoon Bavaricmn. More recently a similar substance was obtained in the Archzean series of Bohemia, and named by F ritsch Eozoon Bolzenuk-mn. A.IEP.IC...—-In. North America Archzean rocks cover a large part of the continent from the Arctic Circle south- wards to the great lakes. They appear likewise, as in Europe, along the central parts of prominent mountain chains, as in the Rocky Mountain range and that of the Appalachians. They have been carefully studied in Canada, where the late Sir W. E. Logan, Director of the Geological Survey of the Dominion, estimated their depth at about 30,000 feet, but neither their top nor their base can there be found. He named them the Laurentian system from their abundant development along the shores of the St Lawrence. They have been divided into two series——(1) a lower formation more than 20,000 feet thick, consisting chieﬂy of granitic, orthoclase gneiss, with bands of quartz- rock, schists, iron-ore, and limestone ; and (2) an 11pper formation fully 10,001) feet thick, composed also, for the most part, of gneiss, but marked by the occurrence of bands of Labrador felspar, as well as schist, iron-ore, and limestone. The upper division has been stated to lie un- conformably on the lower. Mr Selwyn, however, has recently pointed out that this is almost certainly not the case, but that the limestone-bearing series rests conformably upon a massive granitoid gneiss, to which he would restrict the term Laurentian, classing the limestones in the next or Huronian system (Nat. Hist. Soc. Jlontreal, Feb. 1879). In one of the Laurentian limestones of Canada, speci- mens have been found of a remarkable mixture of calcite and serpentine. These minerals are arranged in alternate layers, the calcite forming the main framework of the substance with the serpentine (sometimes loganite, pyroxene, &c.) disposed in thin, wavy, inconstant layers, as if ﬁlling up ﬂattened cavities in the calcareous mass. So different from any ordinary mineral segregation with which he was acquainted did this arrangement appear to Logan, that he was led to regard the substance as probably of organic origin. This opinion was adopted, and the structure of the supposed fossil was worked o11t in elaborate detail by Dr Dawson of Montreal, who pronounced the organism to be the remains of a massive foraminifer which he called Eozoon, and which he believed must have grown in large thick sheets over the sea-bottom. This opinion was con- ﬁrmed by Dr W. B. Carpenter, who from a large suite of additional and better preserved specimens, described a system of internal canals having the characters of those in true foraminiferal structures. (See FORAMINIFERA.) Other observers, notably Profesors King and Rowney of Calway and Miibius of Kiel, have opposed the organic nature of Eozoon, and have endeavoured to show that the supposed canals and passages are merely inﬁltration veinings of ser- pentine in the calcite. In some cases, however, the “ canal- GEOLOGY [vr. sTRATIGRAI‘III(‘AL. system” is not filled with serpentine but with dolomite, which seems to show that the cavities must have existed before either dolomite or serpentine were introduced into the substance. Dr Carpenter contends that the disposition of these passages in his decalcitied specimens is very regular, and quite unlike any mineral infiltration with which he is acquainted. The opinion of the organic nature of Er):00n has been supposed to receive support from the large quantity of graphite found throughout the Archzean rocks of Canada and the northern parts of the United States. This mineral occurs partly in veins, but chiefly disseminated in scales and laminae in the limestones and as independent layers. Dr Dawson estimates the aggregate amount of it in one band of limestone in the Ottawa district as not less than from 20 to 30 feet, and he thinks it is hardly an exaggeration to say that there is as much carbon in the Laurentian as in equivalent areas of the Carboniferous system. He compares some of the pure bands of graphite to beds of coal, and maintains that no other source for their origin can be imagined than the deoxidation of carbonic acid by living plants. In the largest of three beds of graphite at St John he has found what he considers may be fibrous structure indicative of the existence of land-plants. Still further evidence in favour of organized existence during Archzean time in the North American area has been adduced from the remarkably thick and abundant masses of iron ore associated with the Laurentian rocks of Canada and the United States. Dr Sterry Hunt has called atten- tion to these ores as proving the precipitation of iron by decomposing vegetation during the Laurentian period on a more gigantic scale than at any subsequent geological epoch. ‘ Seine of the beds of magnetic iron ranged up to 200 feet in thickness. Large masses also of ha-matite and titanifcr— ous iron, as well as of iron sulphides, occur in the Canadian Archzean series. These great bands of iron ore run southward, and fprm an important feature in the economic geology of the Northern States of the Union. Above the Laurentian rocks in the region of Lake Huron lies a vast mass of slates, conglomerates, limestones, and quartz-rocks, attaining a depth of from 10,000 to 20,000 feet. They are termed Huronian. N o fossils have yet been found in them ; but they must be much younger than the Laurentian rocks, on which they rest uneonformably, and from which they have been in part at least derived. I 1. PA L;EOZOIC. Under the general term of Primary or Palrcozoic are now included all the older sedimentary formations containing organic remains, up to the top of what is termed the Permian system. These rocks consist mainly of sandy and muddy sediment with occasional intercalated zones of lime- stone. They everywhere bear witness to comparatively shallow water and the proximity of land. Their frequent alternations of sandstone, shale, conglomerate, and other detrital materials, their abundant, rippled, and sun-cracked surfaces marked often with burrows and trails of worms, as well as the prevalent character of their organic remains, show that they must have been deposited in areas of slow subsidence, bordering continental or insular masses of land. As regards the organisms of which they have preserved the casts, the Palzeozoic rocks, as far as the present evidence goes, may be grouped into two divisions——an older and a newer :—the former distinguished more especially by the abundance of its graptolitic, trilobitic, and brachiopodous fauna, and by the absence of vertebrate remains; the latter by the number and variety of its fishes and amphibians, the 1 Geology Q/' Canada, 1863, p. 573.