. Relics of primeval life, beginning of life in the dawn of geological time. al cavities and pores havebeen filled with a greenish mineral composed of silica,iron and potash, combined with water (or, chemicallyspeaking, a hydrous silicate of iron and potassium),which is named glanconite from its bluish-greencolour—a name which we shall do well to such compounds, bases of similar chemical pro-perties often replace one another, so that variousglauconites differ somewhat in composition, the ironbeing in part often replaced by alumina or magnesia,and the potash by soda. The combined wa
. Relics of primeval life, beginning of life in the dawn of geological time. al cavities and pores havebeen filled with a greenish mineral composed of silica,iron and potash, combined with water (or, chemicallyspeaking, a hydrous silicate of iron and potassium),which is named glanconite from its bluish-greencolour—a name which we shall do well to such compounds, bases of similar chemical pro-perties often replace one another, so that variousglauconites differ somewhat in composition, the ironbeing in part often replaced by alumina or magnesia,and the potash by soda. The combined water alsodiffers somewhat in its percentage. When minuteshells fossilized in this way are treated with an acidso as to remove the calcareous shell itself, the en-closed silicate remains as a beautiful cast or core, LAURENTIAN LIFE lis representing all the forms of the interior, and anypores that may have penetrated the walls, and alsoperfectly representing the soft gelatinous body ofthe animal which once tenanted the shells (Fig. 24).(See also Fig. 25 at end of chapter.). Fig. 24.—Cast 0/ Cavities of Folystomella in daiuoHUe {)nagnified).After a photograph from Dr. Carpenter, and mounted specimens from his collection. When we examine oceanic sediments of olderdate, we find similar fillings in limestones, chalks,and sandstones of various ages, some of the lattercontaining glauconite so abundantly as to bear thename of greensands, from their colour; and inthese older examples we more frequently find alu- ii6 RELICS OF PRIMEVAL LIFE mina and magnesia occupying a large place in themineralizing silicate. Fig. 24A gives two illustrationsof this—one a crinoidal stem from the Silurian ofNew Brunswick, injected with a silicate of alumina,
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