418 Notices of Memoirs — Prof. LapworWs Address. 



Now if we ask what are these formations which constitute the objects of study 

 of the stratigrapliical geologist, I am afraid that, as in the case of the species of the 

 biologist, no two authorities would agree in framing precisely the same definition. 

 The original use of the term formation was of necessity lithological, and even now 

 the name is most naturally applied to any great sheet of rock which forms a com- 

 ponent member of the earth-crust ; whether the term be used specifically for a 

 thin homogeneous sheet of rock like the Stonesfield slate, ranging over a few square 

 miles ; or generically for a compound sheet of rock, like the Old Red Sandstone, 

 many thousands of feet in thickness, but whose collective lithological characteris- 

 tics give it an individuality recognizable over the breadth of an entire continent. 



When Werner originally discovered that the 'formations' of Saxony followed 

 each other in a certain recognizable order, a second characteristic of a formation 

 became superposed upon the original lithological conception — namely, that of 

 determinate 'relative position.' And when William Smith proved that each of 

 the formations of the English Midlands was distinguished by an asseml)lage of 

 organic remains peculiar to itself, there became added yet a third criterion — that 

 of the possession of ' characteristic fossils.' 



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

 far better expressed by dividing the geological formations into zoological zones, on 

 the one hand, and grouping them together, on the other hand, into chronological 

 systems. 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 the term. 



A 'zone,' which may be regarded as the unit of zoological succession, is marked 

 by the presence of a special fossil, and may include one or many subordinate 

 formations. A system, which is, broadly speaking, the miit of geological time or 

 siiccession, includes many 'zones,' and often, but not always, many 'formations.' 

 A formation, which is the taiit of geological stratigraphy, is a rock sheet composed 

 of many strata possessing common lithological characters. The formation may be 

 simple, like the chalk, or compound, like the New Red Sandstone, but, simple or 

 compound, local or regional, it must be always recognizable, 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 : 1st. The stratified formations 

 due to the action of moving water above the earth-crust. 2nd. The igneous 

 formations which are derived from below the earth-crust. 3rd. The metamorphic 

 formations which have undergone change within the earth-crust itself. W^e 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 off the land into the 

 waters, and an organic set of limestones, chalk, etc., 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, etc., 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 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 of the sea-bottoms 

 and ocean floors of the earth-crust of their time. 



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



