CHAP. VI.] CARBONIFEROUS PERIOD. 77 



Mr. Or. H. Morton 1 has shown that in this district the 

 Limestone is divisible into four stages, and that at Fron 

 only the highest, and 28 feet of the third remain. Near 

 Chirk the latter have thinned out, and only the uppermost 

 grey beds (137 feet thick) are found. That this thinning 

 indicates an island is proved by the white beds coming in 

 again below the grey at Craig Sychdin, seven miles south 

 of Fron, and these continue to form a base as far as 

 Crickheath Hill, when shales belonging to the second 

 stage appear, and at Llanymynech the total thickness of 

 the limestone has increased again to 450 feet. Here the 

 escarpment terminates ; and when Carboniferous strata 

 set in again five miles to the south-east, Coal-measures 

 rest on Silurian, so that the limestones had thinned out 

 in the interval. 



With regard to the westerly extension of the Limestone 

 we are furnished with valuable testimony in the shape of 

 an outlier, faulted down against Silurian shales, near Cor- 

 wen, and no less than twelve miles W.S.W. of the Eglwyseg 

 escarpment. Moreover, the thickness here is still con- 

 siderable, probably about 750 feet, so that the limestone 

 must have extended some distance farther to the west and 

 south of Corwen. It is hardly likely, however, to have 

 reached so far as the Arenig mountains, though it may 

 have run up the valley of the Dee as far as Bala ; the 

 northern flank of the Berwyn mountains probably formed 

 its southern boundary, these mountains forming a pro- 

 montory which stretched north-eastward towards Llan- 

 gollen and Chirk, and separated what may be called the 

 Corwen bay from the Llanymynech bay (see map, fig. 2). 



The absence of the limestone over the Shrewsbury dis- 

 trict indicates another extension of the land area between 

 Llanymynech and Wellington, where the border of the 



1 " The Carboniferous Limestone and Cefn-y-Fedw Sandstone," 1879. 



