496 John Randall — Geology of LinUy Valley. 



that were such depositions took place, they must have been of pre- 

 cisely the same character. 



The two beds, here separated by the grits, and rock and shales 

 described, comprising about forty feet of strata altogether, become 

 highly interesting when considered as the connecting links between 

 two great formations, and as prolific in the remains of those forms 

 of ichthyic life now engaging so much of the attention of palteon- 

 tologists. In his second edition of " Siluria," Sir E. Murchison, 

 describing rocks occupying similar positions, says, "Fourteen years 

 have now elapsed since I proclaimed that these fishes of the Upper 

 Ludlow rock appeared before geologists for the first time as the 

 most ancient beings of their class ; and all the subsequent researches in 

 the various parts of the world over which Silurian rocks have been 

 found to extend have failed to add to or modify this generalization. 

 In other countries, indeed, besides our own, as in America and 

 Bohemia, one or two ichthyolites have been discovered within the 

 pale of Silurian rocks ; but there, as with us, they are merely found 

 on the outer threshold of the system, and very sparingly." 



Whether such beds are evidences of sudden destruction of the 

 finny inhabitants of these early waters, or merely indicate conditions 

 favourable to fish life — like modern feeding grounds on shallow 

 shelving shores — I cannot say ; but leaving the question as to the 

 cause of this accumulation, nothing is more easy than to read by aid 

 of the light which such remains afford the conditions under which 

 these passage-beds themselves were formed. 



First, there was the period of clear water, when lime alone was 

 thrown down ; next, that during which rivers came down charged 

 with sand containing iron, so fatal to fish life ; but with intervals 

 when clear water prevailed, as evidenced by the Corn-stones. 

 Speaking of these sandstones, Sir H. de la Beche, in the Memoirs of 

 the Geological Society, says, " In the country of which Herefordshire 

 forms the chief portion, extending to Shropshire in one direction, to 

 Monmouthshire and Glamorganshire in another, and into Brecknock- 

 shire, Caermarthenshire, and Pembrokeshire in a third, we have an 

 unbroken exposed surface of 2100 square miles of Old Eed 

 Sandstone ; the whole, making due allowance for vertical differences 

 between the upper and lower beds, of the same general character. 

 Taking the average thickness of the mass as it appears from measuring 

 the beds verticallj'' to their out-crop, we should have more than 1500 

 cubic miles of chiefly red-coloured detrital matter, for such it appears, 

 with the exception of a slight amount of the limestones termed 

 Corn-stones. We have hence in this area alone a large amount of 

 detritus accumulated under conditions very different, as regards the 

 admixture of ferruginous matter, from those which preceded it 

 during the Silurian epoch. It is difficult to obtain a clear view, in 

 the present state of our knowledge, of the causes which produced 

 such a mixture of peroxide of iron with the accumulations formed 

 during the lapse of time apparently required for the deposit of this 

 amount of Old Eed Sandstone. After a few sandstone beds with 

 Lingulce, and perhaps some other shells — ^the last struggles, as it 



