676 Professor Sedgwick and R. I. Murchison, Esq., on the 



among some contorted beds not far from the north end of the canal tunnel, 

 we found some thick amorphous bands of dolomite. 



We are aware of the extreme difficulty of conveying a distinct notion of 

 the true aspect of a rock by mere verbal descriptions ; but we may certainly 

 say in general terms, that the great escarpments near Holcombe Rogus re- 

 semble mountain-limestone. And were we to seek for terms of comparison 

 to describe the ordinary appearance of the black culm limestone (as seen be- 

 tween Bampton and Barnstaple, or along the south boundary of the culm- 

 measures), we might compare it to the black beds of shale limestone in Derby- 

 shire, or to some of the thin-bedded earthy limestones at the bottom of the 

 carboniferous series in Craven ; and more correctly still, to the darker varieties 

 of the Irish calp. But we distinctly wish it to be understood, that we use 

 these terms merely as descriptive, and with no view of prejudging the ques- 

 tion respecting the age of the lower culm group. 



In consequence of the contortions in this group (more particularly in its upper 

 calcareous shales) it is difficult to form any correct estimate of its average 

 thickness: we think that on the whole it is much thicker on the southern than 

 it is on the northern boundary of the great trough. On both sides of Laun- 

 ceston the group is chiefly increased by the great expansion of the lower 

 "■rits : and were we to make a transverse section from the lower beds of the 



o 



whole series on the south side of Bren Tor to the northern limit of the cal- 

 careous shales, we must pass, notwithstanding the contortions, through an 

 ascending series which cannot be estimated at less than two or three thousand 

 feet in thickness, and is we think considerably more*. 



* To make the previous descriptions better understood, we subjoin the following section of the 

 lower culm series where it is exhibited on the largest scale. The section commences about two 

 miles north of Tavistock and ends with the culm limestone on the north side of the great west road 

 — a distance of about eight miles. It exhibits the following ascending order : 



1. Lower shales and grits, the latter of very great thickness. 



2. Trap of Bren Tor, part of it recomposed and regularly bedded. 



3. Thin-bedded jointed rock, here and there penetrated by manganese and by trap. 



4. A more slaty culm shale, having two courses of roofing slate. 



5. Black culm limestone, overlaid by roofing slate, and by indurated thin-bedded shale and 

 siliceous bands much jointed, — penetrated irregularly by manganese. 



6. The same series, repeated over again (by a flexure and a fault) on the north side of Lew- 

 water. 



7. Upper limestone, and commencement of the higher division of the culm-series. 



It is possible that the upper limestone may be only a repetition of the lower by a second great 

 fault. The country is ill-exposed ; but there is nothing in its form or in the dips of the beds to 

 indicate such & fault. The thickness from the bottom shales to the first range of black limestone 

 is, we believe, much greater than what is stated in the text. (See PI. LI., fig. 7.) 



