€98 EEPOKT — 1892. 



But these later superposed conceptions of time-succession, and life-type, are 

 far better expressed by dividing the geological formations, on the one hand, into 

 zoological zones, and grouping them together, on the other hand, into chronological 

 si/stems. For in the experience of every geologist he finds his mind instinctively 

 harking back to the bare lithological application of the word ' formation,' and I do 

 not see that any real advantage is gained by departing from the primitive use of 

 Ihe term. 



A zotie, which may be regarded as the unit of palcBontological succession, is 

 marked by the presence of a special fossil, and may include one or many sub- 

 ordinate formations, or only a part of a formation. A system, which is, broadly 

 speaking, the unit of geological successioyi, includes many ' zones,' and often, but 

 not always, many ' formations.' A formation, which is the unit of geological 

 ■stratigraphy, is a rock sheet composed of many strata possessing common litho- 

 logical characters. The formation may be simple, like the Ghalk, or compound, 

 like the New Red Sandstone, but, simple or compound, local or regional, it must be 

 always recognisable, geographically and geologically, as a lithological individual. 



As regards the natural grouping of these lithological individuals as such, fair 

 progress has been made of late years, and our information is growing apace. We 

 know that there are at any rate three main groups : First, the stratified formations 

 due to the action of moving water above the earth-crust ; second, the igneous forma- 

 tions which are derived from below the earth-crust ; third, the metamorphic forma- 

 tions which have undergone change within the earth-crust itself. We know also 

 that of these three the only group which has hitherto proved itself available for the 

 purpose of reading the past history of the globe is that of the stratified formations. 

 Studying these stratified formations therefore in greater detail, we find that they 

 fall naturally in their turn into two sets, viz., a mechanical set of pebble beds, 

 sandstones and clays, formed of rock fragments, washed oft' the land into the waters, 

 and an organic set of limestones, chalk, &c., formed of the shells and exuviae of 

 ■marine organisms. 



But when we attempt a further division of these two sets, our classification 

 soon begins to lose its definiteness. We infer that some formations, such as the 

 Old Red and the Triassic, were the comparatively rapid deposits of lakes and inland 

 seas ; that others, like the Coal Measures, London Clay, &c., were the less rapid 

 deposits of lagoons, river valleys, deltas, and the like ; that others, like our finely 

 laminated shales and clays of the Silurian and Jurassic, were the slower deposits of 

 the broader seas ; and finally, that others, like our Chalk and Greensand, were 

 possibly the extremely slow deposits of the more oceanic deeps. 



Nevertheless, after looking at the formations collectively, there remains no 

 •doubt whatever in the mind of the geologist that their mechanical members are 

 the results of the aqueous degradation of vanished lands, and that their organic 

 members are the accumulated relics of the stony secretions of what once were 

 living beings. Neither is there any possibility of escape from the conclusion that 

 they have all been deposited by water in the superficial hollows — the lakes, the 

 sea-bottoms, and the ocean tioors — of the earth-crust of their time. 



In the life of every individual stratified formation of the mechanical type we 

 •can always distinguish three stages : first, a stage of erosion and transportation, 

 in which the rock fragments were worn oft' the rocks of the higher ground and 

 washed down by rain and rivers to the sea ; second, a stage of deposition and 

 consolidation below the surface of the quiet waters ; and third, a final stage in 

 which the completed rock-formation was bent and upheaved, in part at least, into 

 solid land. In the formations of the organic type three corresponding stages are 

 equally discernible : first, the period of mineral secretion by organised beings ; 

 second, the period of deposition and consolidation; and third, the final period of 

 local elevation in mass. But one and all, mechanical and organic alike, the forma- 

 tions bear in their composition, in their arrangement, and in their fossils, abun- 

 dant and irresistible evidences that they were the products, and that they are 

 the memorials of the physical geography of their time. 



Guided by the principles of Hutton and Lyell, geologists have worked out with 

 great care and completeness the effects of those agencies which rule in the first 



