374 BULLETIN OF THE NEW YORK STATE MUSEUM 



unless they are filled by finer material. The more open and 

 loosely-aggregated the mass, the more porous the stone, and 

 the greater the volume of water, carrying dissolving agents, 

 which can be absorbed, and the larger the surface area 

 which can be attacked by them. The arrangement of the 

 grains also favors or prevents the ingress of these attacking 

 forces. For example, a laminated structure, with its planes 

 along which the flow of moisture or water is facilitated or is 

 easier than in other directions, helps the tendency to exfoli- 

 ation, or the scaling off of thin sheets and laminae. On a 

 larger scale the arrangement of rocks is seen in beds and 

 the movement of subterranean waters is along their bedding 

 or jointage planes. Where the mass is more closely in- 

 terwoven, as it were, the saturation, although equally great 

 in volume, is not onward flowing in given planes and in- 

 creasingly active, and there are no lines of weakness along 

 which the disruptive forces can act so readily. The ideal of 

 strength of structure is that wherein the grains are in close 

 contact, and they are of varying sizes, filling all the space, 

 as in such an arrangement no planes of sjolitting, and, 

 hence, exfoliation, are possible, without cutting across the 

 grains themselves. Hence the density which is observed in 

 some of the conglomerates, where the spaces are well filled 

 by a strong, siliceous cement. All rocks are, however, more 

 or less porous. The particles are not everywhere in actual 

 contact, and there are spaces of greater or less extent be- 

 tween them. The most dense and compact limestones and 

 sandstones, and the crystalline granites and marbles as well, 

 have such interstices. And, in general, the specific gravity 

 is indicative of the degree of porosity in stones of a given 

 class, that is, in sandstone, as compared with sandstone, or 

 granite with granite. It will be understood that the spe- 

 cific gravity of the particles or mineral species composing 

 the rock mass, determines that of the stone. Thus quartz 

 has a specific gravity of 2.65, whereas hornblende and py- 

 roxene rise to 3.3-3.5, and, hence, the specific gravity is 



