228 BROWELL AND KIBKBY ON MAGNESIAN LIMESTONE, ETC. 



From the preceding analyses it is evident that the Magnesian 

 Limestone differs greatly in chemical composition, so far as relates 

 to the proportions of the two carbonates, in its various beds. 

 Putting aside the specimens from the Lower Red Sandstone and 

 Marl-slate (Nos. 1-1 5a), and taking those only of the Magnesian 

 Limestone proper, we find that they range from a limestone con- 

 taining 96*94 carbonate of lime and 1*66 carbonate of magnesia 

 (No. 50), to one containing 42*48 carbonate of lime and 49*86 

 carbonate of magnesia (No. 35). 



There does not seem much doubt that the differences in che- 

 mical composition here indicated are largely due to the action of 

 segregation after deposition. Such a process has evidently taken 

 place in these remarkable concretionary limestones (botryoidal, 

 coralloidal, &c.) of the upper part of the Series, where the hard 

 crystalline portions which assume the forms of grapes, coral im- 

 perfect spheres, &c, are seen to be chiefly composed of carbon- 

 ate of lime, while the softer portion that fills the interstices of 

 the concretions have a large per centage of carbonate of magnesia 

 (Nob. 38 and 39, 46 and 47). Nor is it unlikely that the repeated 

 alternations of crystalline and earthy strata and laminae, often 

 seen in sections of the Upper Limestone (Fulwell, and coast 

 section S. of Marsden), are due to the same cause, and not to 

 differences in the sediment deposited. 



As a rule, it is the friable limestones that contain the greatest 

 proportion of carbonate of magnesia, and the compact and crys- 

 talline limestones that contain the largest excess of carbonate of 

 lime, as will be seen from Table III. 



Hue is a less constant indicator than texture of chemical com- 

 position — though generally the light yellow beds are the most 

 highly magnesian, and those of a darker colour (brown and grey) 

 the most highly calcareous. 



The two carbonates usually form from ninety to ninety-nine 

 per cent, of the limestone. 



Iron, either as an oxide or carbonate, is only present in small 

 quantities. 



Silica and alumina also never attain a large per centage except 

 in the case of the shaly partings in the Lower Limestone (No. 22.) 



