l68 RELICS OF PRIMEVAL LIFE 



even harder and more resisting than the shell 

 itself. 



Serpentine is a mineral which has been produced 

 in different ways. Some igneous or volcanic rocks 

 consist largely of compounds of silica and mag- 

 nesia (olivine, etc.). When these rocks have be- 

 come cold and are exposed to the action of water, 

 they sometimes absorb this and become hydrated, 

 thus passing into a kind of serpentine. When such 

 rocks are pulverized and dispersed as volcanic ash, 

 this falling into the sea may be there hydrated, 

 and may form serpentinous layers, or in a fine 

 paste or in solution may pass into the pores and 

 cavities of shells and other organic things, acting, 

 as we have seen, in the same manner with ordinary 

 glauconite. In like manner serpentine of this 

 origin may form nodules or grains in limestones, 

 in consequence of its particles being aggregated 

 together by concretionary attraction. We have 

 already seen that some comparatively modern so- 

 called glauconites are essentially of the nature of 

 serpentine, and we know that in the old Lauren- 

 tian sea, salts of magnesia and magnesian minerals 

 were abundant, so that serpentinous minerals might 

 play a greater part than they do in the modern 



