82 Mr. T, Sterry Hunt 



The other work before us is Prof. H. D. Rogers' elaborate report 

 on the geology of Pennsylvania, giving the results of the Survey 

 of that State for many years carried on under his direction, and 

 embracing a minute description of those grand exhibitions of 

 structural geology, which have rendered that State classic ground 

 for the student. The volumes are copiously illustrated with maps, 

 sections and fig:ures of ors^anic remains, and the admirable 

 studies on the coal fields of Pennsylvania and Great Britain add 

 much to its value. 



The oldest series of rocks known in America is that which has 

 been investigated by the officers of the Geological Survey of Ca- 

 nada, and by them designated the Laurentian system. It is now 

 several years since we suggested that these rocks are the equiva- 

 lents of the oldest crystalline strata of western Scotland and Scan- 

 dinavia.* This identity has since been established by Sir R. I. 

 Murchison in his late remarkable researches in the north-western 

 Highlands, and he has adopted the name of the Laurentian system 

 for these ancient rocks of Ross, Sutherland, and the Western 

 Islands, which he at first called fundamental gneiss.f These 

 are undoubtedly the oldest known strata of the earth's crust, and 

 therefore off"er peculiar interest to the geologist. As displayed iu 

 the Laurentide and Adirondack mountains, they exhibit a volume 

 which has been estimated by Sir William Logan to be equal to 

 the whole palaeozoic series of North America in its greatest devel- 

 opment. The Laurentian series consists of gneiss, generally 

 granitoid, with great beds of quartzite, sometimes conglomerate, 

 and three or more limestone formations, (one 1000 feet in thick- 

 ness) associated with dolomites, serpentines, plumbago, and iron 

 ores. In the upper portion 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 char- 

 acteristics seem to distinguish the Laurentian series throughout its 

 whole extent, so far as yet studied, from any other system of 

 crystalline strata. It appears not improbable that future researches 

 will enable us to divide this series of rocks into two or more 

 distinct systems. 



* Esquisse Geologique du Canada, 1855, p. 17. 



t Quar. Journal Geol. Society, vol. xv, 353 ; xv. j 215. 



