DENBIGHSHIRE GRITS, NEAR CORWEN, NORTH WALES. 489 



their almost entire absence in the Longmynd and other districts. 

 For the same reason also it mnst bo expected that a great diversity 

 will be shown in the sediments belonging to this period in different 

 areas, and particularly so in proportion as they approached to or 

 were distant from any of the raised parts. The fauna would also 

 in consequence vary considerably. For these reasons it becomes 

 difficult to correlate with any satisfaction all the beds which are 

 found between the Hirnant limestone of the Bala epoch and the base 

 of the Wenlock, which are known in different places chiefly under 

 the name of Llandovery or May-Hill, or Tarannon, and in the area 

 under consideration as the Denbigh Grit and Flag series. The con- 

 clusions, therefore, arrived at in regard to the plant-remains and the 

 geological horizon in which they are found are: — that the age of the 

 beds must be somewhere between the base of the Wenlock and the 

 Lower Llandovery, probably not far from the horizon of the May- 

 Hill beds (Mid Silurian) ; that the plants did not live on the surfaces 

 on which they are now found ; that their position here is an acci- 

 dental one ; that they were not brought from a great distance, as 

 they occur at several horizons ; that the shore-line from which they 

 were derived was towards the south or west ; and that the land 

 areas were chiefly formed towards the close of the Bala epoch. 



If we compare these plant-remains with those discovered in lower 

 Palaeozoic rocks in other areas in this country, we do not find so im- 

 portant an assemblage anywhere so low in position, certainly not at 

 a lower horizon than the Upper Ludlow rocks, and probably not 

 below the Devonian. It is probable also that an equal number of 

 important plants have not been found together at so low a geological 

 horizon in any other part of the world. Those found in the Silu- 

 rian rocks elsewhere are : — the branch of a fern, described by Count 

 Saporta under the name of Eopteris Morieri, discovered by Professor 

 Moriere in the Middle Silurian at Angers, France; the Glyptoclendron 

 of Prof. Claypole, from the Clinton group of Ohio, America ; and the 

 species of Psilophytum, Annularia, and Splwnophyllum described by 

 Prof. Lesquereux, also from the Silurian rocks of Ohio. It is a 

 curious fact that in each of these areas, in Britain, France, and 

 America, the land plants are in a greatly broken condition, and 

 occur in association with a marine fauna. 



Their geological position in each country seems to bear out the 

 view that physical changes were taking place almost contempora- 

 neously in Britain, in parts of the continent of Europe, and in 

 America at this time. These changes, which took place towards 

 and at the close of the Lower- Silurian (Ordovician) epoch, caused 

 land to be formed in each of these areas of greater extent than 

 could have existed since the earliest Cambrian times ; therefore 

 it is probable that unless we find land plants in the lowest Cambrian 

 deposits, we are not likely to meet with them in the intermediate 

 groups, which appear to have been deposited upon each other unin- 

 terruptedly. That there were periods of shallow water, when de- 

 posits were thrown down nearly at an equal rate with the depression, 



