1893. | NEW YORK ACADEMY OF SCIENCES. 101 
and cavities often contain considerable quantities of rock frag- 
ments, in which the proportion 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. 
Relation 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. Brooks} 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 planes 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 
-—— 
* Geology of N. Y., 2d District, p. 37 et seq. 
t American Journal of Science, iii., TV., p. 23. 
