﻿122 NEW YORK STATE MUSEUM 



dency of the whole rock to develop elongate or fibrous aggregates. 

 It is probable therefore that the parting capacity is in places con- 

 siderably increased by the very process that has produced just the 

 reverse results in the more heterogeneous portions of the beds. 



Under a sufficient stress the rock will part most easily along the 

 planes where this foliate or fibrous character is most persistent. 

 Even in these cases, however, it may not indicate that the rock is 

 essentially weak. It simply locates the most vulnerable point in 

 the stone. In many quarries these streaks are so abundant that 

 only thin slabs can be obtained — the disturbances of ordinary 

 quarrying being sufficient to cause parting. The deeper portions of 

 quarries are, however, much less subject to such behavior. In all 

 cases the greater slab development of the exposed portion of the 

 ledge is an ordinary weathering effect, by which the same results 

 are obtained slowly and naturally and more perfectly than can be 

 secured artificially on the fresh material of the same beds. The 

 expansions and contractions of changes of temperature, together 

 with the rupturing effects of freezing water caught in the pores, 

 serve finally to weaken every part of the rock. In this process the 

 prominent reed lines give way so much in advance of the rest of 

 the rock that they develop into true rifts and separate slabs appear. 

 It must be appreciated that these ledges have been exposed an im- 

 mensely long time compared with the probable requirements of any 

 engineering structure, and that this weathering tendency does not 

 mean a speedy disintegration of the freshly quarried blocks. Still 

 it is advisable to avoid as many sources of weakness as possible 

 and one of the ways is to select ledges where the stone does not 

 have a reeding tendency, or in which the reed lines are interlocked, 

 or wavy, or interrupted. These requirements are most fully met in 

 the coarser beds and especially those exhibiting some cross-bedding. 

 Two local quarries meet these demands to a marked degree. 



Strength. The better qualities of bluestone have great 

 strength. Even the reed lines are in many instances stronger and 

 more durable than the regular quality of some other sandstones 

 that are usually considered suitable building material. The secret 

 of this exceptional strength lies in the modifications of texture that 

 have resulted from the alteration and reconstruction of the mineral 

 constituents. The breaking up of the orthoclase feldspar, and the 

 accompanying changes in the ferromagnesian minerals, have fur- 

 nished considerable secondary quartz, which has in part attached 

 to the original quartz grains making them more angular and de- 



