342 EEPORT ON NATIONAL MUSEUM, 188G 



than a limestone (see p. 350). A thoroughly crystalline or non-crystal- 

 line compact aiid homogeneous limestone or dolomite is scarcely, if any, 

 more absorbent than a granite, and hence it is as little liable to injury 

 from freezing. Professor Geikie, in studying rock-weathering as dis- 

 played by the marble tombstones in Scottish cemeteries, observed that 

 the process presented three distinct phases, all of which were at times 

 observable on the same slab. These were (1) superficial solution, caused 

 by the carbonic and sulphuric acids of the atmosphere ; (2) internal dis- 

 integration, accompanied or preceded by the formation of an exterior 

 coat or film of sulphate of lime; and (.3) curvature and fracture. The 

 first phase manifested itself in loss of polish and gradual roughening of 

 the surface, followed by the formation of minute rifts and final rapid 

 disintegration. One case is mentioned in which a stone erected in 1 785 

 became so far decayed as to require restoration in 1803, and at the time 

 of writing (1880) was and had been for some years so corroded as to be 

 entirely illegible. 



The second phase, that of internal disintegration, manifested itself 

 in a peculiar manner. In a number of cases examined it was found 

 that the sulphuric acid brought in contact with the stone by rains 

 had reacted upon the calcium carbonate,. producing a superficial coating, 

 varying in thickness from that of a sheet of paper to a millimeter, of 

 sulphate of lime. This, so long as it remained intact, seemed to protect 

 the stone from other atmospheric influences. On the breaking of the 

 crust, however, it was found that the cohesion of the crystalline gran- 

 ules beneath had been destroyed and the stone crumbled rapidly to 

 sand, the cause of which is attributed largely to mechanical agencies. 



The third phase, that of curvature and fracture, was observed only on 

 thin slabs of marble which had been placed in a horizontal or vertical 

 position and confiued by a frame of sandstone. It manifested itself in 

 the bulging outward of the slab like the bellying of a well-filled sail. 

 In one case examined, that of a slab of marble 30J inches long, 22 ; | 

 inches wide, by three-fourths of an inch thick, which had been thus se- 

 cured against a wall, the slab was found to have escaped from its fasten- 

 ings at the sides, though still held at the top and bottom, and to have 

 bulged outward sufficiently to allow the insertion of the hand and arm 

 between it and the wall at the widest point. It had also expanded 

 laterally so as to be one-half an inch wider in the center than at the 

 ends. The outer surface of the slab where the greatest strain was pro- 

 duced by the bending was filled with minute cracks or rifts, the largest 

 of which were some one-tenth inch in diameter. The cause of the bulg- 

 ing is believed by Professor Geikie to be due to expansion caused by the 

 freezing of water absorbed from rains. * 



Professor Geikie's conclusions from the examination of a large num- 

 ber of cases were to the effect that in all but exceptionally favorable 

 and sheltered localities slabs of marble exposed to the weather in such 



*Geol. Sketches, pp. 170-172. 



