1893.] NEW YORK ACADEMY OF SCIENCES. 101 



and cavities often contain considerable quantities of rock frag- 

 ments, in which the proj^ortion of insoluble constituents is 

 largely increased. Weathered surfaces often show many 

 projecting lumps of silicates, which ultimately become entirely 

 freed by solution of the surrounding rock. 



Ilelalion of Limestone to Gneiss. — From a stratigraphic stand- 

 point one of the most important problems of the region is 

 presented in the relation between the limestone and the gneiss. 

 As is well known, Emmons* regarded the limestone as an 

 igneous rock that had broken through the gneiss, which latter 

 he held to be of undoubted sedimentary origin. Brooksf was 

 led by his studies in Rossie to consider the limestone uncon- 

 formable with the gneiss, and in this opinion the later 

 investigators, with the exception of Hunt, seem to coincide. 



The precise character of the relations between the two forma- 

 tions, however, cannot be definitely settled, until the true nature 

 of the gneiss is determined. If the gneiss is a metamorphosed 

 sedimentary mass, whose foliation and banding are identical 

 with jDlanes of deposition, it must be ascertained whether or no 

 there is discordance between this foliation and banding and the 

 the bedding of the overlying limestone. For several reasons it 

 is difficult to procure exact data bearing upon this point. One 

 of the obstacles is the massive character of the limestone ; 

 another the variability of the gneiss, making it impossible to 

 tell whether or not the limestone is always in contact with the 

 same horizon of this formation. A third difficulty lies in the 

 rarity of contacts between the two formations. Thus, while a 

 general parallelism in the structure of the formations is plainly 

 apparent, it is not easy to decide whether or not there is true 

 conformity. Such a parallelism might readily exist in uncon- 

 formable formations, being the result of two distinct periods of 

 folding in the same direction. But in spite of this general 

 parallelism between the gneiss and limestone, two localities 

 were found that showed a marked discordance, while at several 

 other points this was strongly indicated. 



This discordance between the bedding of the limestone and 

 the foliation and banding of the gneiss may be general, but 

 before it can be called an unconformity between sedimentary 

 deposits the foliation and banding must be proved to be identi- 

 cal with sedimentary bedding. Until this is done two other 

 suppositions must be considered as possible explanations of 

 , . — .14 



* Geology of N. Y., 2d District, p. 37 et seq. 

 t American Journal of Science, iii., IV., p. 23, 



