290 ANNUAL OF SCIENTIFIC DISCOVERY. 



It is of a somewhat coarse and highly crystalline structure, and 

 abounds in iron pyrites, qualities which, according to all practical ex- 

 perience, are not at all recommendatory. In examining this limestone 

 at the quarry, Prof. Johnson found that it was worked wholly below 

 the original surface of the ground. At a few points along the out- 

 cropping edge of the bed, the harder parts of the rock come to the sur- 

 face of the ground, or rise occasionally two feet above it, covered in 

 places with loose granules of the same rock. In some parts it is eroded 

 into sloping channels, lined with skeletons of crystals and their slightly 

 cohering nuclei. On removing the soil the rock is found with alternat- 

 ing peaks and cavities ; its surfaces are more or less deeply tinged 

 with the oxide of iron derived from decomposed iron pyrites, many 

 veins of which traverse it in various directions. Skeleton crystals, 

 'with slightly cohering nuclei, are even more numerous than at the 

 points where the rock crops out at the surface. Adjoining this quarry 

 is another, of similar limestone, but of a finer grain and stronger tex- 

 ture. Of the unfitness even of this latter limestone, which is supe- 

 rior to the alum limestone, for architectural purposes, the Washing- 

 ton Monument at Baltimore is an instance. This Prof. Johnson 

 finds, after a lapse of 21 years from its completion, to be in a state of 

 rapid decay. Fractures extend throughout nearly the whole of the 

 shaft, and in one instance a block has split into three pieces. Some 

 of the fractures extend through 10 or 12 courses of stone, and ascend 

 some forty or fifty feet. These dilapidations have been produced 

 by natural causes. 



When an attempt is made to polish a portion of the alum limestone, 

 portions will occasionally be detected, which, from their softness, ren- 

 der fruitless all attempts to impart lustre to them. Some are so soft 

 as to be readily scratched with the finger-nail. In many parts, little 

 triangular cavities, from which, it appears, the last remnants of solid 

 angles of crystals have dropped out in the operations of cutting and 

 polishing, will easily be discovered by the eye, and slight lines form- 

 ing the boundaries of large crystals are not unfrequently traceable, 

 when the piece is brought into a strong light. A block thus polished 

 will have all its crystals of pyrites, from their superior hardness, left 

 slightly elevated above the surface of the carbonate of lime, which is 

 worn away in polishing. The surface of the polished stone, thus va- 

 riously marked with elevations and cavities, may be used like an en- 

 graver's plate, and will give an impression of its own markings of 

 much interest. The following results were obtained by Dr. C. G. 

 Page, in some experiments made to ascertain the capability of the 

 alum limestone, and various other well-known building materials, to 

 resist a crushing force. The experiments were made on two-inch 

 cubes, with a powerful hydraulic press, so arranged as to indicate 

 accurately the amount of pressure. It was found that the average 

 strength per square inch of fine-grained Maryland marble was 4,481 

 (the figures representing the weight in pounds to a square inch neces- 

 sary to crush a two-inch cube of the material) ; East Chester marble 

 (N. Y.), the material of which the General Post-Office is constructed, 

 3,993; Italian marble, 3,156 ; Patapsco granite, 2,767 ; Seneca sand- 



