Physical Structure and older stratified Deposits of Devonshire. 675 



We were at first doubtful with what rock we were to class the Holcombe 

 Rogus limestone ; but after a second visit in 1836^ when we were better ac- 

 quainted with all the varied characters of the lower culm-measures^ we did not 

 hesitate to place it in that part of the series. For it comes within the strike of 

 the culm-measures; and it contains many beds resembling the limestone at 

 Bampton (which is unquestionably a culm-limestone^ and may be regarded as 

 a connecting link between the ordinary black limestone and that of Holcombe 

 Rogus). It rests on rotten pale-coloured and red shale, with bands resem- 

 bling china stone, exactly like some beds on the line of the wavellite rock above 

 described. Again, it alternates with bands of red unctuous shale, (which be- 

 come indurated and pass into red flagstones,) and with bands of chert; and 

 among its beds in one of the quarries is found a considerable portion of earthy 

 manganese : in all which respects it is analogous to the Bampton limestone! 

 Lastly, in the extreme paucity of its fossils, and their absence from the accom- 

 panying shale, as well as from its general structure, it is entirely unlike any of 

 the limestones subordinate to the older group under the culm series*. Hence we 

 think it certain that this limestone is a part of the lower culm group ; differing 

 from the other culm-limestones in being much more largely developed, at the 

 expense of the shales and flagstones, and, as a natural consequence, differing 

 also in being more pure and crystalline. It is repeated in six or seven distinct 

 ranges of elevated or contorted beds and anticlinal ridges, within a space of 

 three or four miles in length and breadth. In consequence of these replica- 

 tions, it is difficult to conjecture what is the whole number of its beds or their 

 aggregate thicknessf. In the grand anticlinal ridge (about a mile and a half 

 south of Holcombe Rogus) there is a quarry at Canon Leigh that exposes a 

 face of limestone rock about 150 feet thick : it is composed of a series of very 

 thick, jointed, and shattered beds of a dark grayish blue colour, weathering- 

 like mountain-limestone, and alternating with thinner bands associated with 

 hard red shale and chert. Among several of the subcrystalline beds we ob- 

 served what may be considered a characteristic appearance in these quarries, 

 viz. the dissemination of a series of distinct crystals of carbonate of lime 

 through the more earthy base of the rock. It perhaps deserves remark, that 



* We found a few fragments of encrinital stems in the limestone ; but since the preceding de- 

 scription was written, Major Harding has found in it the characteristic Posidonia, which completes 

 the evidence for our conclusion. 



t The several rifl e . ; limestone vary considerably in their strike; some ranging nearly east 

 and west, others N.E. and S.W. Indeed, all the phenomena of this contorted district may be con- 

 sidered as instances of what has been before stated, respecting the great derangement of the older 

 rocks, as they approach the plains of the new red sandstone. 



VOL.V. — SECOND SERIES. 4 S 



