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and foi'ming the ooze of the sea bottom. Then the alkalis 

 released by the decomposition of the felspars, chemically affect 

 the calcareous matter, and naturally lead us to give a paragraph 

 on the subject of marl. 



Marl. — Since Marl and Iron enter so largely into the newer 

 beds of the Middle Lias, we incline to devote a paragraph to the 

 former, reserving the latter for the subject of the Margaritatus 

 Zone. The word Marl is used in various popular senses. We mean 

 by it " a friable earthy variety of calcite, containing a variable 

 proportion of clay." The marl incorporated with the Spinatus 

 sands derives its argile from the decomposition of the felspars 

 of the gneissic or granitic rocks, which the force of the 

 ancient liassic sea was ever grinding down. Disseminated more 

 or less generally through the Spinatus sands, we see finely 

 divided white mica, due to the destruction of the same plutonic, 

 or it may be to metamorphosed rocks ; the mica seems unaltered 

 except in the degree of trituration it has undergone. Whilst 

 the calcareous ingredient of the sands might be not only 

 drawn from the sea-water itself, but also indirectly from the 

 finely comminuted state of shells, crinoids, polypidoms and 

 foraminifera, which were so freely dispersed through the mass 

 of sand, and lent themselves in that state, of course after their 

 emerging from the sea, to the action of acidulated rain-water. 

 Our view would regard this ingredient in the aspect of a fresh- 

 water marl. Nature though is very complex in her operations, 

 and does not restrict herself to the exertion of single forces or 

 modes of action. The water then that percolated so easily 

 through these loosely cohering sands after their emergence, 

 attacked, dissolved and in obedience to thermal and chemical 

 conditions, redistributed the lime either in the form of 

 concretionary calcite, or else in mechanical combination with 

 the argile of the sands, thus forming a true marl. These 

 remarks though general, are pertinent to the topic, in 

 consequence of Marl and Lime constituting such important 

 elements in the petrology of nearly the whole of the Middle 

 Lias deposits. Going from the Marl, a word upon the other 

 constituent is this, that the calcareous element seemed to have 



