78 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 20, 



D. Causes of Variations in the Persistency of Strata. — The facts 

 here stated regarding the thinning- away of the various kinds of 

 strata bear out in a remarkable manner the views laid down by Sir 

 C. Lyell in his ' Principles of Geology.' "When rivers carry down 

 to the sea fragmentary rocks, sands, and mud, of various degrees 

 of coarseness, these materials are brought within reach of marine 

 currents, and are by their means spread over the bed of the ocean. 

 The larger fragments are the first to become imbedded ; while the 

 more minute particles of sand or clay are carried to great distances 

 out to sea, and slowly sink to the bottom, forming shales and clays, like 

 those of the Red Marl, and Lias. Of these phaenomena, the Triassic 

 group produces examples. In Cheshire, where that formation is 

 most fully developed, the Bunter and Keuper are of nearly equal 

 thickness ; but we have seen that the beds of the Bunter, which are 

 formed of siliceous particles, more or less coarse and of a rather 

 high specific gravity, do not reach as far to the south-east (that is, 

 out to sea) as the eastern borders of Leicestershire and "Warwick- 

 shire, where the fine muddy strata of the Keuper are still 500 feet 

 in depth. The case is further illustrated when we compare the 

 limited range of the breccias, or shingle-beaches, with the other 

 beds of the Trias. These are never found to extend far from their 

 parent coast-lines. 



In observing individual beds also, we find a marked connexion 

 between horizontal area and tranquillity of deposition. Those strata 

 exhibiting the agency of local marine currents, which have pro- 

 duced the phaenomena of false-bedding, are invariably inconstant, 

 and, as it were, short-lived; while the evenly bedded layers, in 

 which the fossils (in the case of fossil-bearing rocks) are entire, 

 have much wider ranges. As points of comparison, we may place 

 on the one side the two Mottled Sandstones of the Bunter and the 

 shelly Freestones of the Inferior and Great Oolites, and on the other 

 the Lower Keuper Sandstones, the Ragstones of the Inferior Oolite, 

 the Upper Zone of the Great Oolite, and the Cornbrash. 



§ 6. Apparent South-easterly Attenuation of Carboniferous 

 Series. — I believe that the conclusions here stated regarding the 

 south-easterly attenuation of the Lower Mesozoic rocks are ap- 

 plicable, to some extent, to the Carboniferous rocks of Britain. 

 The Lower Carboniferous rocks are more largely developed in 

 Scotland than in the North of England ; and my colleague Mr. 

 Geikie informs me that the sandstones and shales of the Lothians 

 evince a marked tendency to become thicker towards the north- 

 west. 



Professor Phillips* appears to have arrived at a similar conclusion 

 with respect to the " Yordale Rocks " of Yorkshire • and he has 

 shown how the sandstones, shales, and other sedimentary strata 

 thicken towards the north-west ; while the organically formed 

 limestones appear in greatest force in an opposite direction. I have 

 already alluded to the example in the case of the Warwickshire Coal- 



* Geology of Yorkshire, p. 176. See also Explanation of the Horizontal 

 Sections of the Geological Survey, Sheet 45. 



