No. 104.] 307 



Limestones aistd Marbles. 



lu limestones and marbles, the conditions of durability find causes of 

 destruction, as a general rule, differ little from those of sandstone. 

 There is nevertheless one point of distinction, which may be noted in 

 the outset. In all the marbles and older stratified limestones — that 

 is, of the Silurian, Devonian or Carboniferous age — the want of co- 

 hesion among the particles, or a friable condition of the rock, may be 

 regarded as fatal to its durability as a building stone; while on the 

 other band, as has been observed, some of the friable sandstones 

 harden by exposure to the weather. In the calcareous deposition 

 termed travertin, however, which is a deposit of modern origin, the 

 mass, on first exposure, is soft and friable, and is frequently cut into 

 blocks of the required shape and dimensions by the axe or saw; after 

 being laid up in the wall it hardens and becomes quite indestructible. 

 Some limestones are said to possess this power of hardening upon ex- 

 posure. 



In almost all limestones, as well those which are unaltered as 

 those which have been metamorphosed, and are known as marbles imr 

 excellence, there is a considerable amount of argillaceous matter, 

 either present in seams parallel to the lines of bedding, or dissemi- 

 nated through the mass. In the dark-colored uncrystalline or com- 

 pact fine-grained limestones this matter is evenly distributed through 

 the mass, and, when only in small proportion, produces no noticeable 

 effect. Some of the varieties of this kind of limestone will stand the 

 exposure of a century, without any essential or injurious change. 

 The compact fine-grained blue limestones without seams are therefore 

 among the most durable stones we have. 



In the gray or bluish-gray subcrystalline limestones the argillaceous 

 matter, instead of being distributed throughout the mass, is usually 

 present in tlie form of seams which are parallel to the lines of bedding, 

 or distributed in short interrupted lamiuas. These seams, whether 

 continuous or otherwise, are fatal to the integrity of the stone; and 

 there is scarcely a limestone structure in the country, of twenty-five 

 years' stantling, which is not more or Lss dilapidated or unsightly, 

 from the effects of absorption of water by the clay seams, and the al- 

 ternate freezing and thawing. When laid in the position of the 

 original beds, which is the usual mode, the separation by the clay 

 seam is sU)wer; but when used as posts or pillars, with the lines of bed- 

 ding vertical, the change goes on more rapidly. 



In the dressing of limestone, the tool crushes the stone to a certain 

 depth, and Leaves the surface with an interrupted layerof a lighter color, 

 on which the cohesion of the particles has been partially or entirely 

 destroy<'d; and in this condition the argillaceous seams are so covered 

 and obscured as to be scarcely or at all visible, but the weathering of 

 one or two years usually shows their presence. 



The usual process of dressing limestoue rather exaggerates the 

 cause of dilapidittion i'rom the shaly seams in the material. The clay 

 being softer than the adjacent stone, the blow of the hammer or other 

 tool l)n-aks the limestone at the margin of the seam, and drives for- 

 ward into the space little wedge-shaped bits of harder stone. A care- 

 ful examiuation of dressed surfaces will often show the limestone 

 along the seam to be fractured, with numerous thin wedge-shaped sliv- 



