654 Professor Sedgwick and R. I. Murchison, Esq.^ on the 



a degenerate form, the great upper limestone of South Devon. At Plymouth the strike is very 

 nearly east and v?est ; but following the strata towards the east, we find them gradually deflecting 

 a few degrees towards the north, which necessarily throws the limestone of Berry Head a little to 

 the north of the Plymouth limestone. 



Before we proceed to describe our remaining sections, we may briefly notice 

 some of the characters of the South Devon limestones. 



1. Where the sections are complete, they are interpolated in a great slaty 

 series, and, indeed, very often pass into mere calcareous slates, subordinate to 

 the g-eneral mass: parts even of the Plymouth limestone pass into mere cal- 

 careous slates. Its bottom beds pass into the strata on which they rest : its 

 upper beds, in like manner, are exactly parallel to, and pass into, those by 

 which they are overlaid. Hence, the present inclined position of the calca- 

 reous masses is due to the same causes which produced the present position 

 of the slates. 



2. The want of continuity in the limestones proves, that the causes which 

 produced them acted less generally than the mechanical causes that gave rise 

 to the layers of mud and sand out of which the slate series originated. It is 

 difficult to speculate about the origin of calcareous slates where organic remains 

 are almost wanting ; but many parts of the upper limestone seem to be, in a 

 great measure, due to the action of animals which once inhabited shells and 

 corals. Now, the coral reefs and shell beds of the ancient sea must have been 

 formed at the same inclination with the beds of sand and mud which were 

 contemporaneous with them, and consequently were once nearly level. 



3. There is a strong resemblance in the mineral structure of all the upper 

 limestones of South Devon : at one extreme they are earthy and slaty ; at the 

 other perfectly crystalline. The most crystalline portions seem to have been 

 formed about masses of abraded corals ; and many of the hardest and thickest 

 beds have more or less of a brecciated structure, as may be seen in some of 

 the well-known Devon marbles. A colour derived from red oxide of iron is 

 often irregularly impressed on the beds of limestone ; and as the same colours 

 are found in the next overlying group, as well as in some of the beds, inferior 

 to the great limestone, this circumstance often produces a striking appearance 

 in the range of the deposit through the country. The limestone was probably 

 consolidated sooner than the slate, and hence may have offered planes of 

 greater resistance, during successive periods of elevation. It is to this cause 

 that we attribute the existence of some flexures in the inferior slates, which 

 hardly seem to aff*ect the great overlying masses of limestone, and produce 

 what we regard as a deceptive appearance of discordant stratification. What 

 makes this explanation more probable is the fact, that the thin beds of lime- 



