GEOLOGY OF THE NEW YORK CITY AQUEDUCT 121 



quartz as the most important and abundant. Others probably occur 

 that are less readily differentiated, and among them is kaolin. Occa- 

 sionally a small amount of massive or granular pyrite occurs. There 

 are traces of organic remains, especially plant stems, and the pyrite 

 is most plentiful in association with those beds. 



It seems to be the secondary products largely that give the char- 

 acteristic bluish or greenish color to this stone. Practically all of 

 the iron freed by secondary changes from the ferromagnesian con- 

 stituents has entered into new silicate compounds, especially with 

 the chlorite, which are minutely distributed throughout the whole 

 mass, giving it all a tinge of the characteristic color of these well 

 known products. The same amount of iron in the oxid form 

 would no doubt give as highly colored stone as any of the reds or 

 browns of other familiar types of sandstone. But the tendency to 

 form the sericite-chlorite-quartz aggregate in the rock has also an 

 important bearing on its durability and strength. This is further 

 discussed in a separate paragraph. 



Classification. It is clear that this type of bluestone is a sedi- 

 mentary rock of medium grain, a sand rock or '' renyte." Since 

 the silicates are so predominant in the original composition it may 

 be further identified as a sandstone or a " silicarenyte." But in 

 view of the predominance of the feldspars it should be further 

 designated as an arkose sandstone. And considering the extent to 

 which it has been modified by the development of interstitial sili- 

 cious products and the effect that this has had in perfecting the 

 bond between the grains, the rock may be classified as an indurated 

 arkose sandstone. 



Special structure. A study of the cause of reeding, or the 

 tendency to split into slabs, led to the preparation of thin sections 

 of this structure [pi. 23, upper figure]. It is apparent from them 

 that the reed is strictly a rock structure and that the perfection of 

 the capacity to split along these planes depends wholly upon the 

 abundance and arrangement and size of the elongate and semifibrous 

 grains and the presence of a more than usual amount of original 

 fine or flaky material. Almost universally the reed streaks are 

 darker in color and finer in grain than the average of the rest of the 

 rock. 



In part therefore it is an original character due to the assorting 

 action of water during deposition, finer streaks alternating with 

 coarser ones in accord with ordinary sedimentation processes. But, 

 in addition to that, the subsequent changes that have affected the 

 whole rock have occasionally accentuated the structure by a ten- 



