93 Kev. A. Sedgwick on the Geological Relations and 



joints are formed by the compressed surfaces of the contiguous compound spheres*. Large masses 

 of stone exhibiting on their weathered surfaces the indications of the structure here described, 

 have occasionally been used for building. 



Such are the principal modifications of the central deposit of magnesian 

 limestone. In local descriptions many further details might be wanting : 

 but what has been done may be sufficient to convey a correct notion of the 

 oeneral character and relations of the great system of beds I have been 

 considering-. 



Before leaving the subject, it seems natural to inquire what limits we can 

 assign to the effects of crystalline forces acting upon large masses. If they 

 have produced such striking effects upon a secondary, mechanical deposit, may 

 they not have produced, in a simdar way, still more important modifications in 

 the more ancient and more crystalline stratified rocks? An answer to this 

 question seems essential to a solution of some of the difficulties presented by 

 the ancient zones of schistose rocks which surround the lowest unstratified 

 protuberances of the earth. 



Thickness of the deposit. — To the average thickness of this division of the 

 formation, it is difficult even to make an approximation ; as it not only rests 

 upon an uneven, unconformable surface, but appears also to have been de- 

 posited with much irregularity. Thus at Eldon, Perry Hill, and Thickley, 

 the limestone, covering the coal-strata near the escarpment, is about fifty feet 

 thick : but at Cowndon, the same portion of the limestone covering has lately 

 been proved to the depth of more than 120 feet. The aggregate thickness of 

 the beds in some parts of Durham and Yorkshire must be very considerable. 

 The bore-hole at Hart, though descending to the depth of 312 feet, neither 

 commenced in the highest beds nor descended to the lowest f. Perhaps five 

 hundred feet might be taken as an approximation to the maximum thickness. 



Organic remains and conclusions. — In the preceding details the organic 

 remains which occur in this part of the deposit have been hardly noticed ; 



* The following section, from a part of the quarry of Fulwell Hill, may convey some notion of 

 the complex nature of the deposit, and of the manner in which the spherical concretions are 

 grouped with the other portions of the rock. 



1. At the top of the quarry irregular, cellular beds of dark-grey and brown limestone, with 

 some globular concretions, especially near the lines of stratification : thickness = 10 feet. 

 2. Under the preceding, a bed six or eight feet thick, chielly composed of globular concretions 

 imbedded in ochreous magnesian earth. 3. Thinly laminated beds much mixed with earthy 

 matter, and containing a few spherical concretions : = 4 feet. 4. Earthy cellular beds, with 

 hard, grey, irregular concretions, = 3 feet. 5. Hard, dark brown beds, partly laminated and 

 earthy, = 8 feet. 6. Thin, yellowish grey, slaty beds, = 10 feet. 



In a section from another part of the quarry the details would not be the same, because the 

 different varieties of the rock constantly replace each other. 



t Geol. Trans. First Series, vol. iv. p. 8. 



