Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/339

Rh USE or 1«‘ossILs.] the lapse of many years, no discovery has occurred to alter the established order of succession of these fossils, nevertl1e- less the subdivisions can only be held good for the region in which they have been made. They must not be supposed to be strictly applicable everywhere. Advancing into another district or country where the petrographical char- acters of tlie same formation or system indicate that the original conditions of deposit must have been very different, we ought to be prepared to ﬁnd a greater or less departure from the ﬁrst bserved or what might be regarded as the normal order of organic succession. There can be no doubt that the appearance of new organic forms in any locality l1as been in large measure connected with such physical changes as are indicated by diversities of sedimentary materials and arrangement. The Upper Silurian formations, for example, as studied by Murchison in Shropshire and the adjacent counties, present a clear sequence of strata well deﬁned by characteristic fossils. But within a distance of 60 miles it becomes impossible to estab- lish these subdivisions by fossil evidence. If we examine corresponding strata in Scotland, we ﬁnd that they con- tain some fossils which never rise above the Lower Silurian form-itions in Wales and the west of England. Again, in Bohemia and in Russia we meet with still greater depart- ures from the order of appearance in the original Silurian area, some of the most characteristic Upper Silurian organ- isms being there found far down beneath strata replete with records of Lower Silurian life. Nevertheless the general succession of life fron1 Lower to Upper Silurian types re- mains distinctly traceable. Such facts warn us against the danger of being led astray by an artiﬁcial precision of palzeontological detail. Even where the palzeontological sequence is best established, it rests probably in most cases not merely upon the actual chronological succession of organic forms, but also, far more than is usually imagined, upon original accidental differences of local physical condi- tions. As these conditions have constantly varied from region to region, it must hardly ever happen that the same minute palacontological subdivisions, so important and instructive in themselves, can be identified and paralleled, except over comparatively limited geographical areas. It cannot be too frequently stated, nor too prominently kept in view, that, although gaps occur in the succession of organic remains as recorded in the rocks, there have been no such blank intervals in the progress of plant and animal life upon the globe. The marcl1 of life has been unbroken, onward and upward. leological history, therefore, if its records in the stratified formations were perfect, ought to show a blending and gradation of epoch with epoch, so that no sharp divisions of its events could be made. But the progress has been constantly interrupted ; now by upheaval, now by volcanic outbursts, now by depression. These interruptions serve as natural divisions in the chron- icle, and enable the geologist to arrange his history into periods. As the order of succession among stratiﬁed rocks was ﬁrst made out in Europe, and as many of the gaps in that succession were found to be widespread over the European area, the divisions which experience established for that portion of the globe came to be regarded as typical, and the names adopted for them were applied to the rocks of other and far distant regions. This application has brought out the fact that some of the most marked breaks in the European series do not exist elsewhere, and, on the other hand, that seine portions of that series are much more complete than in other regions. Hence, while the general similarity of succession may remain, different subdivisions and nomenclature are required as we pass from continent to continent. A bed, or limited number of beds, characterized by one or more distinctive fossils, is termed a zone or Iior-izon, and, GEOLOGY 325 as already mentioned, is often known by the name of a typical fossil, as the different zones in the Lias are by their special species of ammonite. Aseries of such zones, united by the occurrence among them of a number of the same species or genera, is called a group. A series of groups similarly related constitute a formation, and a number of formations may be united into a system. The terminology employed in this classiﬁcation will be discussed in the following part. PART 'I.—STRATIGB.APHICAL GEOLOGY. This branch of the science arranges the rocks of the. eartl1’s crust in the order of their appearance, and interprets the sequence of events of which they form the records. Its province is to cull from all the other departments of geology the facts which may be needed to show what has been the progress of our planet, and of each continent and country, from the earliest times of which the rocks have preserved any memorial. Thus from mineralogy and petrography it obtains information regarding the origin and subsequent mutations of minerals and rocks. From dynamical geology it learns by what agencies the materials of the earth’s crust have been formed, altered, broken, upheaved, and melted. From structural geology it understands how these materials were put together so as to build up the complicated crust of the earth. From palzeontological geology it receives in well- determined fossil remains a clue by which to discriminate the different stratiﬁed formations, and to trace the grand onward n1arcl1 of organized existence upon this planet. Stratigraphical geology thus gathers up the sum of all that is made known by the other departments of the science, and makes it subservient to the interpretation of the geological history of the earth. The leading principles of stratigraphy may be summed up as follows :— 1. In every stratigraphical research the fundamental re- quisite is to establish the order of superposition of the strata. Until this is accomplished it is impossible to arrange the dates and make out the sequence of geological history. 2. The stratified portion of the earth’s crust, or geological record, as it has been termed, may be subdivided into nat- ural groups or formations of strata, each marked throughout by some common genera or species, or by a general resem- blance in the type or character of its organic remains. 3. Many living species of plants and animals can be traced downward through the more recent geological forma- tions ; but they grow fewer in number as they are followed into more ancient deposits. With their disappearance we encounter other species and genera which are no longer living. These in turn may be traced backward into earlier formations, till they too cease, and their places are taken by yet older forms. It is thus shown that the stratiﬁed rocks contain the records of a gradual progression of organic forms. A species which has once died out does not seem ever to have reappeared. But as has been already pointed out in reference to Barrande’s doctrine of colonies, a species may within a limited area appear in a formation older than that of which it is characteristic, having temporarily migrated into the district from some neighbouring region where it had already established itself. 4. When the order of succession of organic remains among the stratified rocks has been determined, they become an invaluable guide in the investigation of the relative age. of rocks and the structure of the land. Each zone and formation, being characterized by its own species or genera, may be recognized by their means, and the true succession of strata may thus be conﬁdently established even in a country which has been shattered by dislocation, or where the rocks have been folded and inverted.