DENBIGHSHIRE GRITS, NEAR CORWEN, NORTH WALES. 



485 



In addition to the above-mentioned tolerably well-preserved plant- 

 remains from the shales associated with the grits, there are un- 

 doubted evidences of a still earlier and probably equally important 

 flora in beds of slate at the base of the quarry. Instead, however, 

 of the remains being preserved chiefly in the condition of mineral 

 charcoal, as in the upper beds, they occur here mainly in the state 

 of a very pure anthracite. At the same horizon some large nodules 

 are seen ; and in the centre of these anthracite is also occasionally 

 found, evidently forming the nucleus. From some of the specimens 

 examined, I conclude there can be no doubt that these plants must 

 have been of considerable size ; and the amount of carbon left on 

 some of the surfaces, apparently from a single fragment only, would 

 tend to show, as suggested by Mr. Carruthers, that it must have been 

 derived from vascular plants. There can be little doubt, therefore, 

 that there is in the slates and nodules, even in the so-called Taran- 

 non slates, very clear evidence of a terrestrial flora of considerable 

 importance*. The anthracite, as now found, is usually broken into 

 innumerable small fragments ; but it is perfectly clear that this must 

 be due to changes to which it has been subjected since it was de- 

 posited — changes which also produced induration, cleavage, and frac- 

 tures in the argillaceous sediments. The fissures in the anthra- 

 cite, and in the charcoal in the other beds, are generally filled by a 

 fibrous mineral, which occurs here in some places in considerable 

 abundance. Mr. T. Davis has kindly examined this, and says that 

 it is a " fibrous form of a hydrated magnesian silicate." In other 

 cases the fissures are filled with calcite. 



The difference in the conditions of fossilization in which the 

 remains are now found at the two chief horizons may doubtless be to 

 a great extent explained by taking into consideration the manner in 

 which they were imbedded in the deposits. The thick grit-beds were 

 evidently thrown down rapidly, and covered over the fragments before 

 decomposition had taken place in them to any great extent. The 

 fine muddy deposits which compose the slates were evidently thrown 

 down much more slowly, and in a tolerably quiet sea ; therefore vege- 

 table material resting on the bottom would have time to decompose 

 almost completely before it would be sufficiently covered over by the 

 deposits. To a certain extent the same cause has allowed remains 

 of vascular plants only to be preserved, as cellular ones would be too 

 readily and too completely destroyed to show indications of their 

 presence in such deposits. This is, I believe, the reason why re- 

 mains of Algse are not more frequently found in these older rocks, 

 and why impressions only are seen in most cases. 



The Pen-y-Grlog quarry, where the specimens were obtained, has 



* Since this paper was read I have received from Mr. Phillips, the manager 

 of Pen-y-G-log quarry, to whom I am indebted for much assistance, specimens 

 which show clearly that plant-remains occur in the pale shales below the Den- 

 bigh-grit series in the same state of preservation as in the beds at the top of 

 the quarry. These shales also contain numerous calcareous nodules. 



Q. J. G. S. No. 147. 2 k 



