GEOLOGY. 273 



has been estimated by Sir William Logan to be equal to the whole 

 paleozoic series of North America in its greatest development. The 

 Laurentian series consists of gneiss, generally granitoid, with great 

 beds of quartzite, sometimes conglomerate, and three or more lime- 

 stone formations (one, one thousand feet in thickness), associated with 

 dolomites, serpentines, plumbago, and iron ores. In the upper por- 

 tion of the series an extensive formation of rocks, consisting chiefly 

 of basic feldspars without quartz and with more or less pyroxene, is 

 met with. The peculiar characters of these latter strata, not less than 

 the absence of argillites and talcose and chloritic schists, conjoined 

 with various other mineralogical characteristics, seem to distinguish 

 the Laurentian series throughout its whole extent, so far as yet stud- 

 ied, from any other system of crystalline strata. It appears not im- 

 probable that future researches will enable us to divide this series of 

 rocks into two or more distinct systems. 



Overlying the Laurentian series on Lake Huron and Superior, we 

 have the Hurouian system, about ten thousand feet in thickness, 

 and consisting to a great extent of quartzites, often conglomerate, 

 with limestones, peculiar slaty rocks, and great beds of diorite, which 

 we are disposed to regard as altered sediments. These constitute 

 the lower copper-bearing rocks of the lake region ; and the im- 

 mense beds of iron ore at Marquette and other places on the south 

 shore of Lake Superior have lately been found by Mr. Murray to be- 

 long to this series, which is entirely wanting along the farther east- 

 ernoutcrop of the Laurentian system. This Huronian series appears 

 to be the equivalent of the Cambrian sandstones and conglomerates 

 described by Murchison, which form mountain masses along the west- 

 ern coast of Scotland, where they repose in detached portions upon 

 the Laurentian series. 



Besides these systems of crystalline rocks, the latter of which is 

 local and restricted in its distribution, we have along the great Appa- 

 lachian chain from Georgia to the Gulf of St. Lawrence a third series 

 of crystalline strata, which form the gneissoid and mica slate series 

 of most American geologists, the hypozoic group of Professor Rogers, 

 consisting of feldspathic gneiss, with quartzites, argillites, micaceous, 

 epidotic, chloritic, talcose, and specular schists, accompanied with 

 steatite, diorites, and chromiferous ophiolites. This group of strata 

 has been recognized by Safibrd in Tennessee, by Rogers in Pennsyl- 

 vania, and by most of the New England geologists, as forming the 

 base of the Appalachian system, while the Canadian geologists main- 

 tain that they are really altered palaeezoic sediments, and superior to 

 the lowest fossiliferous strata of the Silurian series. 



In regard to the interesting question respecting the commencement 

 of life on the earth, Mr. Hunt says : The recognition beneath the Si- 

 lurian and Huronian rocks of forty thousand feet of sediment, analo- 

 gous to those of more recent times, carries far back into the past the 

 evidence of the existence of physical and chemical conditions, similar 

 to those of more recent periods. But these highly altered strata 

 exclude, for the most part, organic forms, and it is only by applying 

 to their study the same chemical principles which we now find in 

 operation that we are led to suppose the existence of organic life 

 during the Laurentian period. The great processes of deoxidation in 



