4SS 



NA TURE 



[StrriiMHEK 2 1. 1S93 



from those which now exist. After reviewing all the evidence 

 at my disposal, I must, however, admit that the coarser as well 

 as the finer deposits of the earlier periods appear to be more 

 complex in composition than those of the later. The grits of 

 the Palaeozoic formations, taken as a whole, contain more fel- 

 spar than the sandstones of the Mesozoic and Tertiary forma- 

 tions, and the slates and shales of the former contain more 

 alkalies than the clays of the latter. This statement will hold 

 jjood for the British Isl^s, even when allowance is made for 

 the enormous amount of volcanic material amongst the older 

 rocks — a phenomenon wliich I hold to be of purely local sig- 

 nificance — but I strongly suspect that it will not b: found to 

 apply universally. In any case, it is not of much importance 

 from our present point of view. All geologists will admit that 

 denudation and deposition were taking place in pre-Cambrian 

 times, under chemical and physical conditions very similar to, 

 if not identical with, those of the present day. 



There is, however, one general consideration of more serious 

 import. Additions to the total amount of detrital material are 

 now being made by the decomposition of igneous rocks, and 

 there is no douot that this has been going on during the whole 

 period of time represented by our stratified deposits. It follows, 

 therefore, as a necessary consequence that strict uniformitarian- 

 iim is untenable, unless we suppose that igneous magmas are 

 formed by the melting of sediments. 



So far we have been dealing with the characters of sedimen- 

 tary rocks as seen in hand-specimens rather than with those 

 which depend on their distribution over large areas. Thanks 

 to Delesse {" Lithologie du Fond des Mers." Paris, 1871) and 

 the officers of the Challenger Expedition (" Report on Deep- 

 sea Deposits," 1891), an attempt has now been made to construct 

 maps on which the distribution of the sediments in course of 

 formation at the present time is laid down. It is impossible 

 to exaggerate the importance of such maps from a geological 

 point of view, for on the facts which they express rests the 

 correct interpretation of our stratigraphical records. Imperfect 

 as is our knowledge of the sea-beds of former geological periods, 

 it is in many respects more complete than that of the sea-beds 

 of the present day. The former we can often examine at our 

 leisure, and follow from point to point in innumerable ex- 

 posures ; the latter are known onlyfrom a few soundings, often 

 taken at great distances apart. ^ An examination of such im- 

 perfect maps as we have raises many questions of great interest 

 and importance, to one of which I wish to direct special atten- 

 tion — not because it is new, but because it is often overlooked. 

 Th-; boundary lines separating the distinct types of deposit on 

 the-e maps are not, of course, chronological lines. They do 

 not separate sediments produced at different times, but different 

 sediments simultaneously forming in different places. Now, 

 the lines on our geological maps are usually drawn by tracing 

 the boundary between two distinct lithological types, and, as a 

 natural consequence, such lines will not always be chronological 

 lines. It is only when the existing outcrop runs parallel with 

 the margin of the original area of deposit that this is the fact. 

 Consider the case of a subsiding area — or, to avoid theory, let 

 us say an area in which the water-level rises relatively to the 

 land — and, for the sake of illustration, let us suppose that the 

 boundary separating the districts over which sand and mud are 

 accumulating remains parallel to the old coast-line during the 

 period of deposition. This line will follow the retreating coast, 

 so that if, after the consolidation, emergence, and denudation 

 of the deposits, the outcrop happens to be oblique to the old 

 shore, then the line on the geological map separating clay and 

 sand will not be of chronological value. That portion of it 

 which lies nearer to the position of the vanished land will 

 represent a later period than that which lies further away. If 

 such organisms as ammonites leave their remains in the different 

 deposits, and thus define different chronological horizons with 

 approximate accuracy, the imperfection of the lithological 

 boundary as a chronological horizon will become manifest. It 

 is not that the geological map is wrong. Such maps have 

 necessarily to be constructed with reference to economic con- 

 siderations, and from this point of view the lithological boun- 

 daries are of paramount importance. They are, moreover, in 

 many cases the only boundaries that can be actually traced. - 



^ Suess, Das Antlitz der Erde, BJ. II., s. 267. 



^SeeS.S Buckman, " On the Cotteswjid, Midford, and Yeovil Sands," 

 Qnari. Journ. Geol. Soc, vol. xjv. (1889), p. 440 ; and the same author," On 

 the So-called Upper Lias Clay of Down C\\Sa," Quart Journ. GcoL S/7C., 

 vol. xlvi. (1890), p. 518. Also J. Starkie Gardner, " On the Relative Ages 

 of the American and the English Cretaceous and Eocene Series," GeuL 

 Mas (1884), p. 492. 



NO. 1247, VOL. 48] 



The geological millennium will be near at hand when we can 

 construct maps which shall represent the distribution of the 

 different varieties of sediment for each of the different geo- 

 logical periods. All we can say at present is that increase of 

 knowledge in this direction tends greatly to strengthen the 

 uniformitarian hypothesis. We can see, for example, that 

 during Triassic times marine conditions prevailed over a large 

 part of what is now the great mountain-belt of the Euro- 

 Asiatic continent, whilst littoral and terrestrial conditions 

 existed in the north of Europe ; and we can catch glimpses of 

 the onward sweep of the sedimentary zones during the great 

 Cretaceous transgression, culminating in the widespread deep- 

 sea ' conditions under which the chalk was deposited. 



We turn now to the igneous rocks. It is no part of my pur- 

 pose to treat in detail of the growth of knowledge from an 

 historical point of view, and to attempt to allot to each observer 

 the credit due to him ; but there is one name that I desire to 

 mention in this connection, because it is that of a man who 

 clearly proved the essential identity of ancient and modern 

 volcanic rocks by the application of precise petrographical 

 methods at a time when there was a very general belief that the 

 Tertiary and pre-Tertiary rocks were radically distinct. 

 I need hardly say that I refer to Mr. Samuel Allport.' 

 He wrote at a time when observers in this country had 

 to prepare their own sections, and those who, like myself, 

 have had the privilege of examining many of his slides scarcely 

 know which to admire most — the skill and patience of which 

 they are the evidence, or the conciseness and accuracy of his 

 petrographical descriptions. His papers do not occupy r large 

 number of pages, but they are b.ased on an amount of observa- 

 tion which is truly surprising. The general conclusions at 

 which he arrived as to the essential identity of ancient and 

 modern igneous rocks are expressed with the utmost confidence, 

 and one feels, after going over his material, that this confidence 

 was thoroughly justified. It is curious now to note that the one 

 British champion of the distinctness of the Tertiary and pre- 

 Tertiary rocks pointed to the difference between the Antrim 

 and Limerick traps. These traps differ in exactly the same 

 way as do the corresponding Tertiary and pre-Tertiary conti- 

 nental rocks, with this important difference. On the Continent 

 the ophitic structure is characteristic of the pre-Tertiary rocks, 

 whereas in the north of Ireland it is a marked feature of those 

 of Tertiary age. We see, therefore, that the arguments for the 

 distinctness of the two sets of rocks derived from the two areas, 

 based in both cases on perfectly accurate observations, neutralise 

 each other, and the case hopelessly breaks down as regards the 

 basalts and dolerites. 



In this country it is now generally recognised that, when 

 allowance is made for alterations which are necessarily more 

 marked in the earlier than in the later rocks, there is no im- 

 portant difference either in structure or composition betweea 

 the rhyolites, andesites, and basalts of the Paleozoic and 

 Tertiary periods. Bat identity of structure and composition 

 may in this case be taken to imply identity as to the physical 

 conditions under which the rocks were produced. We are thus 

 led to picture in our minds long lines of vo'canoes fringing the 

 borders of Palaeozoic continents and rising as islands in the 

 Palaeozoic seas. Then, as now, there issued from the craters of 

 these volcanoes enormous masses of fragmental material, a large 

 portion of which was blown to dust by the explosive escape <rf 

 steam and other gases from the midst of molten rock.; 

 and then, as now, there issued from fissures on their flanks v»St 

 inasses of lava which con.solidated as rhyolite, andesite, and 

 basalt. We may sum up the case as regards the volcanic rocks 

 by saying that, so long as observations are confined to a limited 

 area, doubts may arise as to the truth of the uniformitarian 

 view, but these doubts gradually fade away as the area of 

 observation is extended. There are still some outstanding 

 difficulties, such as the apparent absence of leucite lavas amongst 

 the Palaeozoic formations ; but as many similar difficulties have 

 been overcome in the past, it is improbable that those which 

 remain are of a very forujidable character. 



So far we have been- referring to rocks formed at the surC»06 

 of the earth under conditions similar to those now in operation. 

 But there are others, such as granite, gneiss, and mica-schist, 



1 Theodor Fuchs, " Welche Ablageriingen haben wir als Tiefseellil- 

 dungen zu betrachten?" Neues Jahrbuch f. Miner. &c. Beilage, Band 

 II., p. 487. 



2 "Tertiary and Palseozoic Trap-rocks," Gcol. Mag. (1873), p. <9* 

 " British Cart)oniferou3 Dolorites," Quart. Journ. Geol. Soc, vol. XXK. 

 (1874), p. 529: "Ancient Devitrlfied Pitchstones," &C. QitaJ't. Jottr*' 

 Geol. Scc.f vol. xxxiii. (1877), p. 449. 



