GEOLOGICAL EXPLORATIONS AND LITERATURE. 65 



Mvtomiirphism. — None of the oiij;iiiaI ilcposifs now icinaiii pi-ecisely in their 

 primitive eomlitioii, tliouj;li only a portion of tlieni have l)eeii so transformed that the 

 original state is not clearly discernible. Tlie limestone was somewhat compacted and 

 rendered more crystalline, and scattered crystals of tremolite were formed by the 

 union of lime and magnesia with silica — in other words, w(Te generated from a some- 

 what silicious i)ortion of tlie limescouc. Tlie great sand deposits were transformed 

 into (iuartzite, but, for the most part, the original grains and pebbles still remain 

 unobliterated, wliilt^ in some instances line laminations and beautiful lipple and rill 

 marlvs are excellently preserved, bearing the most unecpiivocal testimony to their 

 acjueous origin. The iron ores associated with the (juartzites and silicious schists are 

 now found largely in the form of magnetite or derivations from it. If they did not 

 originally exist in that state (and they probably did not), they were doubtless trans- 

 formed into it at this time of general nietamori>liism. Probably some of the more 

 massive iron deposits in association with clay anil carbonaceous schists, as those 

 of the Commonwealth and Florence mines, were only compacted and dehydrated. 

 Certain substances that accumulated incidentally with the sand of tlie series now 

 constitute accessory minerals scattered througli the (juartzite, as i)yrohisite, novae- 

 nlite, mica, and others. The various liner silts, clays, and mixed sediments were 

 changed to slates and schists. In short, the whole series Avas hardened, compacted, 

 and in some measure chemically transformed and crystallized. The changes in 

 these respects, however, were rarely equal to those of the preceding Laurentian 

 revolution, 



Dinturbance of beds.— In respect to attitude, great changes took place. Beds 

 which lamination, ripple marks, and other characteristics show to have been essen- 

 tially horizontal when formed are now found arched and tilted at high angles. In 

 the Penokee region the strata stand at angles varying from '20° to upwards of 80°. 

 In the Menominee region they were warped and folded in a still more striking manner, 

 and stand at various angles, according to situation. In central Wisconsin, instead of 

 close folds, inunense arches were formed. The Baraboo quartzite ranges are but the 

 insignificant remnant of the mnth side of an arch of gigantic dimensions, which 

 swept upward to an altitude approaching, if not surpassing, the highest existing ele- 

 vations.' Similar broad arches were formed on the Avestern side of the Laurentian 

 island. (Pp. 89-90.) 



(3.) As to an interval between Hurouian ami Keweeuavs^an : 



Betrtveen the Huronian and Keweenawan periods an interval of moderate extent 

 a]tpears to be indicated by the fact that the beds of the latter repose unconformably 

 upon those of the Ibrmer. The amount of this unconformity is, in -Wisconsin, but 



' For Fig., see vol. Ii, p. 506. 

 MON XIX 5 



