﻿Vol. 6l.~] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxvii 



were laid down as sand in one tract and mud in the other were still 

 supplied : in that ease, the micaceous sand would be overlain by one 

 not containing mica, and the same would occur with the mud. In 

 the case used for purposes of illustration, the presence of mica would 

 be of use as a means of correlation, while the bulk of the material 

 composing the rocks would not, for sand was being formed in one 

 place while mud was being formed in another. 



The above imaginary case is used to emphasize the importance of 

 taking into account all the lithological characters of rocks, and 

 not merely those which appear to be dominant, when studying 

 strata with a view of classifying them. We are so accustomed to 

 consider the calcareous, argillaceous, or arenaceous characters of 

 sediments as of primary importance, that we are apt to overlook 

 the significance of other characters, which, in some cases at any 

 rate, may actually be of greater import than the composition of the 

 rock. To illustrate this, we may take note of a few cases. 



The British Jurassic rocks above the Lias contain limestones of 

 very different characters, but in these the frequency of oolitic 

 structure is so great as to have justified the use of the term ' Oolites ' 

 for them, although this structure is of course not always present, 

 and is also frequent in British limestones of other date. Still, 

 the common occurrence of oolite in rocks of Jurassic age over a 

 wide region, indicates that some physical conditions were suffi- 

 ciently prominent in those times to give rise to the structure over 

 our area in an exceptional degree. 



The Devonian rocks of North- Western and Central Europe contain 

 an abundance of calcareous deposit. In these tracts, white crystal- 

 line limestones are particularly abundant in the beds of Middle 

 Devonian age and of the earlier Upper Devonian age ; while, in the 

 later Upper Devonian rocks, nodular earthy limestones of a prevalent 

 grey colour are especially frequent. 



A very interesting case is presented by certain muddy deposits of 

 the Silurian Era. In Bohemia, the beds Eel of Barrande contain 

 faunas of Llandovery, Tarannon, Wenlock, and Lower-Ludlow ages. 

 The strata of the three first-mentioned periods are, as a whole, 

 remarkably similar in lithological characters, though containing 

 different faunas ; whereas, in Britain, the graptolite-bearing rocks 

 of these ages differ one from the other more markedly. But in 

 Bohemia, as in Britain, Scandinavia, and Prance, the shales which 

 contain the Wenlock fauna are marked by the frequent occurrence 

 of large elliptical concretions, often having Orthocerata or clusters 



