126 VETERINARY HYGIENE 



The hardest varieties of sandstones are greatly in demand for 

 paving, copes, steps, foundation-blocks, &c. Sandstones vary in 

 colour, even in one quarry, the shade depending upon the amount 

 and condition of the oxide of iron which it contains. Common 

 colours of sandstone are white, grey, brown, yellow and red. 



All sandstones should be built on their natural bed, that is to say, 

 with their line of cleavage horizontal. Stones set on their edge 

 weather very rapidly, particularly when the line of cleavage is 

 parallel with the direction of the wall, stones being so placed, flaking 

 off with each succeeding spell of rain and frost. Stones built in 

 arches should be set on edge, with the plane of cleavage at right- 

 angles to the face and soffit of the arch in order that the layers 

 composing the stone may present their faces to the line of pressure. 

 In selecting a sandstone, the reputation of which is unknown, it is a 

 safe guide to look for bright, clean and sharp appearance in the 

 texture when the stone is broken. A dull, earthy appearance in any 

 stone indicates liability to decay. 



Limestone is calcium carbonate in combination with iron, silicum, 

 magnesium, &c. Well-known examples of limestones are (1) 

 Portland Stone, obtained from the Upper Oolite series of sedi- 

 mentary rocks in the island of Portland ; and (2) Kentish Rag-stone. 

 This latter is a compact limestone of a bluish tint, and is chiefly 

 used for rubble building, paving setts, road-metal, &c. It is mixed 

 with a soft calcareous sandstone, porous and perishable, which may 

 be found adhering to it. It is very difficult to dress, and sometimes 

 contains nodules of iron, which cause rust stains to show on the 

 stone after exposure; (3) Bath-stone. This stone varies in colour 

 from white to creamy-yellow. When first quarried it is soft and 

 moist. It may be cut out by a saw, and is easily worked for fine 

 carving. After exposure to the atmosphere it hardens, and is very 

 durable. 



SLATES. The chief varieties of slates obtainable are (1) Welsh 

 slates, which have a good cleavage and may be split into very thin 

 metal; and (2) Westmoreland slates, which are hard, tough and 

 very durable, in a variety of colours presenting a very pleasing 

 appearance. 



Scottish slates from Ballachulish, Easdale, Aberfoyle, and a few 

 other quarries are very durable, but are thick and heavy. The 

 weathering properties of a slate depend upon its non-porosity and 

 the absence of white iron pyrites. A good slate should give out a 

 sharp ring when struck. It should stand trimming and holing 

 without fracturing. Slates may be tested by weighing them before 

 and after twenty-four hours' immersion in water. Any slate with 



