30 NEW YORK STATE MUSEUM 



ate to their fineness. The apparent exceptions to this relation of 

 grain to strength are numerous, but they are possibly accounted for 

 by variations of interlockment and cementation between the par- 

 ticles. 



An important element in the strength of some rocks is con- 

 tributed by the interlockment of the particles, an arrangement 

 which acts upon the general structure like hair in a mortar. This 

 is exemplified best of all by the diabases in which the feldspar in 

 lathlike crystals is embedded in a matrix of pyroxene, olivine and 

 magnetite, so as to exert the utmost resistance to both tension and 

 compression. A similar effect may be produced by prismatic horn- 

 blende and pyroxene crystals in the syenites and gabbros or by 

 the mica scales in granites. A dovetailing of the mineral particles 

 contributes to the strength of some marbles and granites. The 

 grains have irregular or indented outlines instead of smooth, 

 rounded borders and are molded upon each other in the closest 

 form of interlockment. 



A uniformity of texture with the minerals spaced after a regular 

 pattern is an advantage both from the standpoints of appearance 

 and of weathering qualities. It is essential for rocks that are to 

 be subjected to abrasion and wear. 



Color. Little significance attaches to color as a guide to the 

 intrinsic merits of building stone. Within narrow limits it may 

 indicate something in regard to the relative state of weathering but 

 a change of color such as may be brought about by oxidation of 

 iron or 'bleaching of carbon compounds on exposure to the air does 

 not necessarily mean a deterioration in strength. From commercial 

 considerations, however, color ranks among the very important 

 qualities and has much to do with the favor which a stone wins in 

 the market. This is especially true of architectural stone for use 

 in our larger cities. There is a certain prevailing taste apparent 

 in the selection of stone with reference to color which finds illustra- 

 tion in city architecture of different periods. At present, the 

 taste seems to incline toward the very lightest colors, white or 

 light gray, often to the exclusion of shades which are much better 

 adapted for service in the surroundings. The employment of white 

 marbles and very light granites for structures in manufacturing 

 districts or for railroad stations seems inappropriate as it is un- 

 necessary. 



The colors found in rocks are too varied to be individually dis- 

 cussed or explained. It may be said that the principal coloring 

 agents are iron and carbon, the former for the igneous class and 



