Murchisoris Silurian System. 



223 



presence of peculiar organic remains, the first strata of 

 which are connected by a gradual advancement in the num- 

 ber and characters of their fossil contents with newer de- 

 posits. Some of these beds were known, others were either 

 not known, or mistaken for rocks connected with, or belong- 

 ing to the coal measures. 



The upper Silurian, or Ludlow formation, consists of red 

 and yellow compact micaceous sandstones, bluish grey 

 mottled limestone, and slaty impure argillaceous slates con- 

 taining lime and sandy particles. The upper stratum of 

 Silurian rocks, where it dips beneath the old red sandstone, 

 is covered by the remains of large fish bones, shells, and 

 coprolites, called the bone bed. The remains contained in 

 this bed partake equally of those forms, (chiefly fishes,) 

 which are found in the upper Silurian, and lower old red 

 system. 



Below these strata, the rocks generally lose the appear- 

 ance of sandstone, and contain more calcareous matter, 

 which is mixed up mechanically in an argillaceous paste.* 

 The strata are usually thin bedded, but are not so compact 

 as to answer for flagstones. The surface of the beds pre- 

 sent a waved undulating appearance, like the ridges and 

 furrows occasioned by the rippling action of waves, to which 

 the appearance is partly due. Mr. Murchison, however, 

 thinks some of the transverse markings have been occasion- 

 ed by animals, such as lived on sandy shores. The animals 

 whose remains characterise these upper beds, are Leptcena 

 lata, t. vii. fig. 15, Cypricardia amygdalina, t. vii. fig. 3 

 Orbicula rugata, t. vii. fig. 19, and Avicula lineata, t. vii. 

 fig. 18, which also occur in the lower beds of the old red 

 system. 



* These stones are used for building, but they do rot answer well, 

 unless when used immediately after extraction from the quarry, when 

 they require to be laid horizontally in the direction of their slaty 

 laminae. 



