146 DR. THOMAS STEERY HUNT ON THE 



bleudic gneiss, and a band of crystalline limestone appearing a little farther to the east, 

 on Harlem River. A quarter of a mile to the west of this ridge, in Mount St. Vincent, 

 is seen a distinct type of highly micaceous gneiss and mica-schists, and similar rocks are 

 exposed at intervals in the western part of the island, as far south as Fifty-ninth Street. 

 Farther eastward, in the southern part of Central Park, just above Fifty-ninth Street, the 

 numerous rock-exposures are all of similar mica-schists and micaceous gneisses, often at 

 moderate angles. They include endogenous granitic veins, occasionally presenting in 

 their structure a marked bilateral symmetry, and sometimes transverse, but at other times 

 interbedded. Several perched blocks here found are of similar endogenous granite, and 

 are apparently boulders of decomposition, left in the subaerial decay of the rocks of the 

 region. These micaceous rocks are unlike those of Laurentian areas, but, on the contrary, 

 closely resemble those of the White Mountains and of Philadelphia which I have called 

 Montalbau, and are like the younger gueissic series of the Alps and the Scottish High- 

 lands. I, therefore, as long ago as 18Y1,™ noticed these rocks as belonging to this 

 younger series, and have since expressed the opinion that the Laurentian " of Manhattan 

 Island appears to be overlaid in parts by areas of younger gneisses and mica-schists, the 

 remaining portions of a mantle of Montalbau."^' It is, however, by an error for which I 

 am not responsible, that in Macfarlane's " Geological Railroad Guide," in 18*78, the Montal- 

 bau of Manhattan Island has been represented as extending upward along the Hudson 

 River Railroad by Spuyten Duyvil, Yonkers, Tarrytown and Siugsing, as far as Croton, 

 before meeting the Laurentian of the Highlands. There appears to be, however, an outlier 

 of Montalban rocks at Cruger's Station, just above Croton, and there may be others in 

 various parts of Westchester County. 



§ 188. It has been deemed necessary to notice thus at length, in this connection, 

 Dana's resuscitation of the ancient views of Mather, for two reasons : first, because therein, 

 both the Lower Taconic rocks and various crystalline rocks just noticed, are supposed by 

 him to be contiguous portions of the same Cambrian and Ordovician (Lower Silurian) 

 sediments in diiferent stages of transformation ; and secondly, because the manner in which 

 the names of the brothers Rogers are cited to Dana in conjunction with that of Mather is 

 such as to lead the reader to the false conclusion, that those eminent geologists supported 

 Mather's hypothesis of 1843 as to the Cambrian and Ordovician age of these same crystal- 

 line rocks, as well as of the Lower Taconic series; which latter view, as we have shown, 

 W. B. Rogers repudiated a few years later, in 1851 and again in 1860. 



§ 189. The rise and fall of the doctrine of regional metamorphism, which is but an 

 extravagant development of the Huttouian hypothesis of the origin of crystalline rocks, 

 forms a curious chapter in the history of geology. I have elsewhere related the early 

 application of this doctrine to the crystalline rocks of Mont Blanc by Bertrand, about 1*797, 

 and its subsequent restatement by Keferstein in 1824, until it was taken up and popu- 

 larized by Lyell, Murchisou, and various continental geologists, so that the view became 

 generally accepted that the gneisses and mica-schists of the Alps are but altered secondary 

 and tertiary strata. The story of the refutation of this hypothesis for the Alps by the 



'"' President's Addres.s before the Amer. Assoc- Adv. Science, 1871, in Cliem. and Geol. Essays, pp. 248 and 

 197. 



^' Smithsonian Report for 1883, Progress of Geology. 



