432 SPECIAL GEOLOGY. 



stone is found to combine the varied qualities so much 

 desired by the architect, but so seldom found in the same 

 material; uniting the softness and facility of working of 

 the oolite above, with the hardness and compact texture 

 of the more crystalline rocks below ; while the magnesia 

 which it contains is unfavourable to vegetable growth. 

 Qualities thus valuable induced the members of the 

 commission for selecting stone for the new houses of 

 Parliament, to give the preference to this material over all 

 others which they had the opportunity of inspecting. It was 

 remarked that in this stone, the carbonates of lime and of 

 magnesia exist in nearly equal proportions. In many 

 instances, this substance presents singular examples of 

 spheroidal structure, induced, it is conceived, after the 

 deposition of the stratified mass. 



OBGANIC REMAINS. "We have already observed, that 

 where certain strata resemble in mineral structure the 

 beds above, the fossils which they contain often present 

 an affinity with the rocks beneath; and, in the present 

 instance, those of the Permian partake rather of the 

 types of the carboniferous group below, than of the new 

 red sandstone above, the shells being of the genera Spirifera 

 and Producta ; while the fish are remarkable for that pecu- 

 liarity of structure in the tail called heterocercal, in which 

 the vertebral column extends into the upper lobe, whilst, in 

 homocercal fishes, the tail is either single or bilobed, and 

 the spine does not extend into either. (Figs. 293, 294). 



FIG. 293. Heterocercal ; Shark. FIG. 294. Homocercal ; Herring. 



On this peculiarity of the tail in the sturgeon and shark, 

 Dr. Buckland remarks that the prolongation of the vertebral 

 column is calculated to sustain the body in an inclined 

 position, with the head and mouth nearest to the bottom. 



