316 Geological Society : — Prof. K Sedgwick and Mr. Murchison 



the other rocks with which they are in contact ; hence whatever may 

 be their age, the Culm Measures are the newest stratified deposits de- 

 scribed in this memoir. Along their northern boundary they rest on the 

 highest group described in Chap, ii j and on their southern boundary they 

 partly rest on the granite and partly on the oldest slate rocks of Devon 

 and Cornwall. Hence they cannot form (whatever be the mineralo- 

 gical appearance) a true passage into the different schistose inasses on 

 which they rest. Again, they are overlaid by no rocks older than the 

 new red sandstone ; their age can therefore only be determined by 

 their structure and organic remains. 



The authors then describe sections of the Culm series in great detail, 

 showing its enormous thickness and endless contortions ; the anti- 

 clinal lines generally ranging with the strike, or nearly east and west. 



For convenience, the series is divided into two groups. 



The lower is made up of dark carbonaceous shales, sandstones, 

 micaceous and siliceous flagstones, and calcareous shale, here and 

 there containing subordinate beds of black limestone. All the Wa- 

 vellite of Devon is found in this lower group. These beds are beau- 

 tifully brought out both along the northern and southern boundaries. 



The upper group is made up of an indefinite alternation of shales 

 and sandstones, variable in structure, but generally rather thin- 

 bedded : commonly, the shales are considerably indurated and re- 

 semble '{ grey wacke," but in other places they are soft and earthy 

 like ordinary coal-shale. 



The sandstone bands vary much in texture j there are large quar- 

 ries of them not to be distinguished from coarse coal grits j very 

 rarely they puton a conglomerate form -, mostfrequently they are close- 

 grained, but even in that respect do not differ much from the grit- 

 stone bands in the carboniferous system of a part of South Wales. 

 Pyrites abounds and ironstone is occasionally found associated with 

 the shale and sandstone. 



Carbonaceous stains and impressions of plants occur in many 

 parts of this great formation, and thin laminae of anthracite are com- 

 mon in both the upper and lower group ; but large masses of 

 anthracite and beds fit to work for use are only found in the upper. 

 The authors then describe the Culm works in the neigh- 

 bourhood of Bideford, where three beds have been worked : the best 

 is stated to average nearly four feet in thickness, while in some places 

 it swells out to twenty feet, and in others contracts almost to nothing. 



The plants differ essentially from those found in the older rocks, 

 but are all identical with those species which are most abundant in 

 the coal-fields of the central counties of England and in the South 

 Welsh basin, among which Cyperites bicarinaia, Neuropteris cordata, 

 N. gigantea, Pecopteris lonchitica, and P. abbreviata are perhaps the 

 most widely distributed. In their lithological aspect also, and in 

 containing vast quantities of small sedge-like vegetables, these 

 culm-bearing strata of Devon are undistinguishable from the coal 

 measures of Pembrokeshire. No animal remains have been disco- 

 vered in them, nor in the underlying sandstone (millstone grit), in 

 which negative features these rocks further coincide with those of the 

 Pembrokeshire coal field. 



