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



2. A great zone of slates, more or less calcareous, and with subordinate 

 beds of limestone : the lower portion containing the Ashburton bands ; the 

 upper containing the great Plymouth and Tor Bay limestone. The upper 

 portion also contains occasional beds of red slate and sandstone ; which some- 

 times appear under the limestone, and sometimes seem to take its place along 

 the lines of strike. 



3. A great arenaceous group, containing many beds of red and variegated 

 sandstone. 



4. A group, with many courses of soft, glossy slate ; sometimes indurated, 

 and used for roofing-slate ; alternating with coarse quartzose bands ; without 

 any subordinate beds of limestone, and apparently without fossils. These 

 four groups complete the ascending series, and are, together, of an enormous 

 thickness. 



5. Lastly; a crystalline group of micaceous and chloritic slate, unlike any 

 other formation in North or South Devon; not co-ordinate with any of the 

 groups above described ; probably of metamorphic structure, and due to some 

 obscure axis of elevation, which has passed near the southern promontories of 

 the county, and interrupted the succession of the regular ascending series. 



Conclusions. 



1. Comparing together the regions of North and South Devon above de- 

 scribed, we find many points of resemblance, and some points in which their 

 structure is remarkably contrasted. For example, trap rocks abound on 

 some portions of the southern sections, but are never found in the northern. 

 Again, in the northern region, a true transverse slaty cleavage is constantly 

 developed among all the finer deposits ; while in the southern, these cleavage 

 planes are almost wanting, and nearly all the roofing slates are obtained 

 from laminae parallel to the stratification. Yet in South Devon, systems of 

 parallel joints are as common as in North Devon. What physical conditions 

 have led to this remarkable difference of structure we do not at present 

 inquire. 



2. Judging both from the mineral structure and organic remains of the 

 two regions, we are justified in affirming that neither of them exhibits any 

 very old class of slate rocks. For example, none of the groups, either of the 

 northern or southern sections, are to be compared with the rocks of Snowdonia 

 or the green slates of Cumberland ; without including in our comparison the 

 still lower group of Skiddaw Forest. 



3. The preceding details show that there are many points of physical re- 



