R. PUMPELLY — MEMORIAL OV T. STEKKY HUNT. 383 



II. Norian, the formation of which was only possible after the crenitic 

 lixiviation had exhausted a large part of the accessible primary mass of 

 much of its silica in the forms of orthoclase albite and quartz, so that 

 the succeeding secretions furnished the less acid silicates, as labradorite 

 and andesite. 



III. Arvonian; a stratified scries of rocks including petrosilex, and 

 quartziferous porphyry associated with beds of quartzite, micaceous 

 schists, great beds of hematite and more rarely layers of crystalline 

 limestone. This series he places between the Laurentian and the 

 Huronian, stating that he is unable to fix its exact relation to the 

 Norian. 



IV. Huronian. — The shrinkage originating in the removal from the 

 primary mass of the material to form the preceding three series caused 

 eruptions from the underlying basic mass, so that extensive areas, both 

 of the crenitic acid rocks and of the eruptive basic rocks, were exposed 

 to subaerial decay. 



In this decay the acid crenitic rocks gave up their alkalies, leaving 

 residual clays, while the basic rocks yielded their lime and magnesia. 

 The alkaline and magnesian carbonates introduced a new factor into the 

 history of the rocks, for, reacting upon the calcium-chloride of the 

 primeval sea, this produced Lime, carbonate and alkaline and magnesian 

 chlorides. Thus a magnesian sea was formed. The reaction of the 

 magnesian salts of this sea upon the petrolitic matters (lime silicates) of 

 the continued crenitic secretions brought into the sediments a vast 

 amount of magnesian silicates, giving a distinctive character and color 

 to the resulting schists. 



V. Montalban. — The Huronian required for its formation a magnesian 

 sea, caused by the subaerial decay of both crenitic and especially of 

 eruptive basic rocks on one hand, and by the continued addition of 

 crenitic lime-silicate secretions contributed in an advanced stage of 

 lixiviation on the other. The building up of the Montalban series of 

 fine-grained gneisses, granulites, mica-schists and schists abounding in 

 aluminous silicates of the Andalusite type presupposes the comparative 

 absence of magnesium from the seas. Here the gneisses are of purely 

 crenitic origin, and the schists are derived mainly from the products of 

 subaerial decay of the older crenitic rocks, the resulting clays, still car- 

 rying a portion of their alkali, with or without the aid of crenitic secre- 

 tions, yielded by diagenesis, muscovite, quartz and the simple aluminous 

 silicates. 



VI. Taconian. — This great series of quartzites, limestones, hydromica- 

 schists and argillites, according to Dr Hunt, marks a stage of diminished 

 energy in the process. In the schists he sees apparently the products 



