202 [Assembly 



particles of sand simply cohering among themselves, or by the inter- 

 vention of some argillaceous, ferruginous, or calcareous matter act- 

 ing as a cement; or lastly through a partial solution and cementa- 

 tion of tlie siliceous particles themselves. In the latter case, and 

 where the mass is pretty purely siliceous, the process may have gone 

 so far as to give a vitreous rock knov?n as quartzite. In many cases, 

 however, the siliceous or arenaceous deposits present great in- 

 equalities of texture, from the aggregation of coarse particles or 

 small pebbles among the finer materials, always to the injury of 

 the strength and durability of the mass. Under certain other con- 

 ditions, these mixtures become crystalline rocks of various character. 



2. The clay, or argillaceous matter by itself or with a small ad- 

 mixture of silica, and often more or less of carbonate of lime, becomes 

 a slate or shale rock, but quite unfit for building stone ; and as a 

 general rule, any rock in which argillaceous matter predominates is 

 unfit for a durable building stone. 



3. Carbonate of lime and magnesia, or the former alone, consti- 

 tutes extensive beds of solid and durable stone, but which is often 

 deteriorated by the presence of argillaceous matter. In many lime- 

 stones, the mass consists of an aggregation of fine particles which 

 have been deposited in the form of a fine calcareous mud. Other 

 and often very extensive beds are visibly composed of the debris of 

 shells and other organic bodies, cemented together by the finer par- 

 ticles of calcareous mud, or often by the partial solution of calcareous 

 matter. Under the influence of subsequent conditions, these simple 

 njeehanical aggregations of calcareous matter, or the calcareous mag- 

 nesian deposits, become crystalline marbles of various colors. 



In the purely siliceous stones, or quartzites, the mass is too hard 

 and brittle for easy working or comely shaping of the pieces ; an 

 admixture of clay or argillaceous matter being essential to the possi- 

 bility of working stone whose basis is silica. When, however, this 

 argillaceous material becomes excessive, the stone is liable to rapid 

 disintegration from the action of the weather. While the silica 

 absorbs but an extremely small quantity of water, the clay will absorb 

 largely ; and this, on freezing, will destroy the stone more or less 

 rapidly. Some of the argillaceous sandstones, on drying in a hot 

 sun and then being suddenly wetted, will crack and crumble into 

 pieces. The same effect is often produced by the sudden freezing 

 of large blocks which have been freshly quarried, and which still 

 retain their water of absorption. 



When the argillaceous matter is evenly and intimately mingled 

 with particles of silica or quartz, and not in too large proportions, 

 the stone will last along time, and will disintegrate but slowly; but 

 when the argillaceous material is in seams or laminm of deposit, it 

 is far more injurious, and every such seam in a block of stone must 

 sooner or later lead to its destruction. The manner of this is very 

 simple. The clay seam absorbs water, and, holding it while freez- 

 ing, the seam expands ; if disintegration does not immediately 

 follow, the seam is widened so that it admits more water on the next 



