I06 NEW YORK STATE MUSEUM 



several miles. Exactly how thick it is can not be determined, but 

 it is at least several hundred feet. It fills the whole width of the 

 valley at one of its widest points, fully a quarter of a mile wide, 

 and, although folding may have caused considerable duplication of 

 strata, it is difficult to avoid an estimate of more than 500 feet. The 

 only other occurrence in the Highlands region comparable to it is the 

 Franklin limestone in ,New Jersey which has a still greater develop- 

 ment, perhaps more than 2000 feet. The only other limestone 

 formation of similar size in the region is the Inwood limestone, 

 farther south, which attains a thickness of at least 750 feet, and has 

 a very extensive distribution from the Highlands to the sea. If the 

 Manhattan-Inwood series is really of Grenville age, then this Sprout 

 brook occurrence may be simply an outlier of the Inwood, as was 

 assumed at one time by the senior author, and may thus represent 

 the much sought extension of that series toward the north. 



Although these two limestones may be traced to within a few 

 miles of each other, their characteristics are so noncommittal 

 that it is not considered fully proved that they are the same. There 

 is no question, however, that the conditions of Grenville time were 

 such as to favor occasional developments of limestone of great 

 thickness, and it is also certain that shales and sandstones or arkoses 

 and sediments of considerable variety were laid down in an appar- 

 ently conformable series of great but undetermined thickness. 



This must have occupied some part of Huronian time, a period 

 of great sedimentary development. But the Huronian was a great 

 complex and, in some regions, is represented by several series of 

 formations with unconformities between them. To which of these 

 the Grenville of this area belongs is quite unknown. 



Grenville metamorphism. Subsequent history to the end of 

 Precambrian time includes metamorphism, igneous intrusion, and 

 erosion. Some of the steps are clear enough to warrant definite 

 statement. Many others are so obscured by succeeding modifica- 

 tions that a complete history can not be unraveled from this region. 

 Doubtless, however, a better history than we now present can be 

 written when the full meaning of some of the confused structures 

 and other criteria can be determined. 



For example, it is still a point of great uncertainty how much of 

 the extensive and profound metamorphism of certain formations is 

 due to regional dynamic melaniorphism rather than to contact influ- 

 ences, or whether it is possible by igneous intrusion alone to produce 

 a recrystallization and a structural habit so nearly identical with 



