GEOLOGY OF THE NEW YORK CITY AQUEDUCT ItQ 



exposed places exhibit the character so prominently only because 

 of the weathering effect, which develops the natural tendency. This 

 general conclusion is borne out by the well known practice of quar- 

 rymen of the district of splitting the larger blocks into slabs of the 

 required thickness by wedges driven along certain streaks that are 

 known as " reeds." A reeding quarry is one that has this capacity 

 well developed, and it is this character in part that has made the 

 *' bluestone " or " flagstone " of New York an important factor in 

 the production of the United Sta^tes for a great many years. 



For large size dimension stone where great stress is involved it 

 is evident that this structure would not be desirable. These definite 

 planes of weakness reduce the general efficiency. A little observa- 

 tijn however shows that there are some outcrops and an occasional 

 quarry where the more massive blocks do not split well. From the 

 necessities of the industry these have been avoided or but meagerly 

 developed. In some cases of this kind the sedimentation is of the 

 cross-bedded type with somewhat interlocked laminae. If the grain 

 is coarse such varieties resist splitting with great success. The 

 thickness of such beds varies from a few feet to 25 feet or even 

 more without prominent interbedding of shale layers. 



Stratigraphy. These are the sandstones, flags and shales 

 known as the Hamilton, Sherburne and Oneonta formations belong- 

 ing to the Devonic period. The strata of the immediate vicinity of 

 this examination belong to the Sherburne subdivision, but no at- 

 tempt to differentiate the formations was made. Structurally and 

 petrographically the dift'erent formations are not distinguishable in 

 this area. On the market the stone from either is known generally 

 as " Hamilton flag " or " bluestone." 



Economic features 



There are hundreds of quarries in this general region. Nearly all 

 are small, and are worked on a small scale without machinery. The 

 product is almost wholly thin slabs of the flagstone type. This is 

 supplemented by a small amount of somewhat more massive char- 

 acter, dressed for window sills ; and a very limited output is of 

 dimension stone of larger size. The general lack of suitable me- 

 chanical devices and transportation facilities are the chief reasons 

 for the limited output of the last named grades. 



Petrography 



The basis of this discussion is a microscopic examination of sev- 

 eral thin sections made of the different types of rock from the 



