SHELL MARLS OF THE MAGNESIAN LIMESTONE. 



49 



Red Sandstone north of the Severn, Ness Cliff, &c., (see section PL 29. fig. 9. 1 ). The 

 central or most calcareous part of this ridge, has been quarried through a thickness of 

 about thirty feet, the beds dipping at an angle of 15°. The strata consist of angular 

 fragments of compact, cream-coloured limestone (the "kernels" of the workmen) in a 

 reddish, sandy, calcareous matrix, in which small cavities occasionally occur, lined 

 with crystals of dolomite. Limestone containing magnesia, is abundant in some 

 beds of the mountain or carboniferous limestone at Llanymynech and Porth-y-wain, 

 distant only a few miles, and that rock being of older date, may have supplied many 

 of the inclosed materials, and much of the cement of this conglomerate. Some frag- 

 ments of limestone, occasionally as large as a man's head, and forming a part of the 

 conglomerate, have, however, been derived from the breaking up of a peculiar fresh- 

 water limestone which is intercalated between the seams of coal in the adjacent carbo- 

 niferous tracts of Coedway, Pontesbury, &c. (See Chapter 5.) Besides the calcareous 

 fragments, there are small, rounded pebbles of white quartz and other ancient rocks. 

 These additional materials become more abundant as the ridge sweeps round to Loton 

 Park, and they are well exposed in a section on the high road from Shrewsbury to 

 Llanymynech, where the strata dip 8° to 10° to the east. The calcareous layers, though 

 here subordinate to sandstone, reappear at intervals through a thickness of at least 

 120 feet, and overlie the thick-bedded red sandstones which form the base of the 

 whole system. This section at Loton (PI. 29. fig. 9.) is therefore a clear and convincing 

 proof of the true position of the dolomitic conglomerate, since it is evidently separated 

 from the underlying coal of Coedway by the Lower Red Sandstone of Pecknell. 



In the chief lime quarries of the Alberbury ridge, were formerly some slight mining 

 trials, occasioned by finding a few thin strings of copper ore. These strings were in 

 the vertical joints of the rock, ranging from south-south-west to north-north-east, or 

 nearly at right angles to the direction of the strata. The sides of these joints are 

 usually faced with upright bands of hard sandstone, termed " Burrs," which cut 

 through the strata and are enemies of the lime-burners. 



Although it has been stated in the previous pages, that no remains of shells have yet 

 been detected in the overlying members of the New Red System of England, a consider- 

 able number of curious and unpublished species, have recently been discovered at Man- 

 chester, in beds of variegated marl 2 . These shelly marls are considered by Professor 

 Sedgwick to lie beneath the upper and central members of the New Red System; and 

 Professor Phillips, who has recently worked out in some detail the relations of the strata 

 in the environs of Manchester, is of the same opinion. In a letter to myself, he 



1 This calcareous conglomerate of Alberbury has been fully described by Professor Sedgwick, and recognised 

 by him as the true equivalent of the magnesian limestone, and its relations to the lower red sandstone established. 

 Geol. Trans, vol. ii. p. 399. 



2 Mr. Leigh and Mr. Binney of Manchester discovered these shells, as announced in a communication which 

 they forwarded through myself to the Geological Society. 



