STRATA OF SHROPSHIRE AND DENBIGHSHIRE. 25 



Stigmarian rootlets in the underclay. Along the strike of the strata 

 there is, at Carribou Island, a thicker coal-seam. Some of the shales 

 contain concretions of limestone which sometimes form a nearly conti- 

 nuous bed ; and analyses show that some of the limestones are highly 

 magnesian. These are overlain by a great mass of red and grey 

 sandstones, chiefly red (group 4), which, in their turn, are covered 

 without apparent unconformability by Triassic strata. Dr. Dawson 

 thinks that the strata of the section bear such strong points of re- 

 semblance to parts of the European Permian, both in their mineral 

 character and organic remains, that they may be fairly termed Permo- 

 Carboniferous. 



The Permian strata are sometimes divided into upper, middle, 

 and lower groups, and somtimes into upper and lower only. In 

 my grouping I have, as will be seen, adopted the tripartite divi- 

 sion of the strata. This threefold division is, I think, more true 

 to nature. For while each of the groups 2, 3, and 4 does, in some 

 of the sections, bear resemblances to certain portions of the other 

 groups in other sections, yet, taking the sections altogether and 

 looking at them comprehensively, each group has one or more 

 characteristic features by which it may be distinguished from the 

 others. 



Thus the lowest division, group 2, is characterized by the prevalence 

 of conglomerates and breccias, by the preponderance of grey over red 

 sandstones, by the large proportion of the fragments of Cambrian and 

 Silurian rocks in its breccias, by the greenish colour of many of those 

 breccias, as well as by the plant-remains which have been found 

 in most of the sections, and which (in sections 7, 8, 17, 19, 20, as 

 well as in Russian sections corresponding to section 1) have been 

 numerous enough to form thin coals. By these features this group 

 may be distinguished from the sandstones and marls of group 4. 



Group 3 is well marked by the presence of calcareous matter, 

 which, usually mixed with magnesia, is more or less interstratified 

 with white, red, and variegated marls, the calcareous matter being 

 most abundant in the north-eastern counties of England. In none 

 of the sections of any thickness are red sandstones and trappean-like 

 breccias present in any force. This group is also characterized, in 

 sections of any thickness, by the presence of shales with plant-remains. 



Group 4 is distinguished by the prevailing dull red colour of its 

 sandstones and marls, together with the massiveness of the former 

 and the thinness of the latter, bj r the infrequency of its breccias and 

 their dissimilarity to the prevailing character of those of group 2, 

 and also by the great dearth of carbonaceous matter and the rarity 

 of plant-remains, although, occasionally, solitary specimens of the 

 latter are found which show a relationship, more or less remote, 

 existing between this and the groups below. 



If now we fix our attention on the Ifton section, 11, and compare 

 it with recognized Permian sections on either side of it, we shall see 

 how strikingly in each group it resembles them and possesses their 

 characteristic features. 



We see how in group 1 it has a series of upper coal-measures inter- 



