78« BEPOET — 1893. 



records. Imperfect as is our knowledge of the sea-beda 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 exposures ; the latter are known only from a few 

 soundings, often taken at great distances apart. ^ An examination of such imper- 

 fect maps as we have raises many questions of great interest and importance, to 

 one of which I wish to direct special attention — not because it is new, but because 

 it is often overlooked. The boundary lines separating the distinct types of deposit 

 on these maps are not, of course, chronological lines. They do not separate sedi- 

 ments produced at different times, but different sediments simultaneously forming 

 in different places. Now, the lines on our geological maps are xisually 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 sepa- 

 rating 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 chrono- 

 logical horizon will become manifest. It is not that the geological map is wrong. 

 Such maps have necessarily to be constructed with reference to economic considera- 

 tions, and from this point of view the lithological boundaries are of paramount 

 importance. They are, moreover, in many cases the only boundaries that can be 

 actually traced.- 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 geological periods. All we can say at present is 

 that increase of knowledge in this direction tends greatly to strengthen the uni- 

 formitarian 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. 



Wo turn now to the igneous rocks. It is no part of my purpose 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 appli- 

 cation of precise petrograpbical 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 ob- 



' Suess, Bus Antlitz der Erie, Ed. 11., s. 267. 



' See S. S. Buckman, ' On the Cotteswold, Midford, and Yeovil Sands,' Quart. 

 Joiirn. Geol. Soc, vol. xlv. (1889), p. 440 ; and the same author, ' On the So-called 

 Upper Lias Clay of Down Cliffs,' Quart. Journ. Geol. Soc, vol. slvi. (1890), p. 518. 

 Also J. Starkie Gardner, ' On the Relative Ages of the American and the English 

 Cretaceous and Eocene Series,' Geol. Mag., 1884, p. 492. 



" Theodor Fuchs, ' Welche Ablagerungen haben wir als Tiefseebildungen zu 

 betrachten ? ' Neues Jahrluch f. Miner., &c., Beilage, Band II., p. 487. 



^ 'Tertiary and Palseozoic Trap-rocks,' Geol. Mag., 1873, p. 196; 'British Car- 

 boniferous Dolerites,' Quart. Jourti. Geol. Soc, vol. xxx. (1874), p. 529 ; ' Ancient 

 Devitrified Titchstones,' &c., Quart. Journ. Geol. Soc, vol. xxxiii. (1877), p. 449. 



