OEIGIN OF CEYSTALLINE EOCKS. 63 



dichroite or iolite-gneiss, aud the occasional occurrence of iolite in the younger or Montal- 

 ban gneisses of New England. 



§ 125. The predominance of micaceous schists of the muscoA^tic type in the upper por- 

 tions of the Montalbau, marks the growing change in the conditions of the process which 

 gave rise to the indigenous crystalline rocks, a process continiied with many modifica- 

 tions, and with diminished energy, through the subsequent period of the Taconiau. 

 This was marked by the deposit of quartzites, limestones, aud argillites, and also 

 by the intercalation of schistose beds, characterized by an abundance of damourite 

 or related micaceous minerals, as well as by the presence of matters apparently felds- 

 pathic, which seldom take upon themselves the characters of well-defined species, 

 though found transformed by subaërial decay into a form of kaolin, aud in some instances 

 apparently assuming the state of an imperfect gneiss. These Taconiau schists, which 

 require careful chemical and microscopic study, also include serpentine, talc, pyroxene, 

 epidote and garnet. The ajipearance in paleozoic argillites of crystals of rutile, of tour- 

 maline, and of staurolite, indicates a latter stage of that condition of things which 

 marked the crenitic process of pre-paleozoic times, and made possible the formation of the 

 whole vast series of primitive and transition crystalline schists which we have sought to 

 include under the names of Laurentian, Norian, Arvonian, Huronian, Montalbau and Taco- 

 niau — designating in their order the upward succession of these great groups from the 

 fundamental granitoid gneisses (here included in the Laurentian) to the dawn of paleozoic 

 time. The Arvonian or petrosilex group intervenes between the Laurentian and the Hixro- 

 nian. The peculiar characters of the Norian, and its localization to some few limited 

 areas in Euro]3e and North America, make it difiicult for us, as yet, to define its precise 

 relations to the Arvonian. The Norian, howcA^er, like the Arvonian, probably occupies a 

 horizon between Laurentian and Huronian. Much time may pass, and many stratigra- 

 phical studies must be made, before the precise relations of the Huronian and the succeed- 

 ing Montalbau can be defined. It seems probable, in the present state of our knowledge, 

 that the Montalbau series, though of great thickness, was, in many cases, deposited over 

 areas where the Huronian had never been laid down. Notwithstanding the great geograph- 

 ical extent and the importance of these two series, neither can claim that universality 

 which apparently belonged to the primitive granitic stratum ; a universality soon inter- 

 rupted by the uplifting of portions of dry land, an event which preceded Huronian time. 



§ 126. That the production of large quantities of similar pectolitic silicates, in regions 

 remote from exotic rocks, was continued from the Pre-Cambrian to far more recent times 

 is evident, from the presence of a considerable deposit of serpentine among the horizontal 

 Silurian dolomites of Syraciise, New York, of which the writer has elsewhere recorded 

 the history,'"* and also from the well-known beds of sepiolite foi;nd with opal in the 

 tertiary dolomites of the Paris basin.**' The recent amorphous zeolitic deposits in 

 tertiary sandstone in Switzerland (§ 94), and the compounds referred in the foot-note to 

 § 104, should not be forgotten in this connection. 



Whether the silicates brought from below by crenitic action were directly separated as 

 feldspars, as crystalline zeolites, or as gelatinous precipitates to be subsequently changed 



i"» Trans. Eoy. Soc. Can., vol. i., part 4, pp. 174-177. 



'* Hunt, on the Dolomites of the Paris Basin, 1860. Amer. Jour. Sci., xxix., p. 284. 



