PROCEEDTNGSi OF THE POLYTECHNIC ASSOCIATION. 765 



into bricks. They are all silicates of alumina, varying in quality 

 according to their mineral composition, but the quantity of alumina 

 necessary to form a sound brick is exceedingly small, and the useful 

 proportions are almost indefinite. Gypsum and alahaster are pure 

 crystalline sulphates of lime, in somewhat dift'erent mechanical 

 conditions. They are neither of them in the state in which the 

 stone is originally deposited. They are found sometimes in veins 

 and sometimes in bands -or nodules of large size. Among the very 

 useful materials in the countries where it is abundant, should be 

 mentioned soapstones (silicates of magnesia), which, in their quality 

 of resisting fire, are excelled by none. America yields them very 

 largely, and at a moderate cost. They are found in Cornwall, but 

 large blocks for building cannot be obtained of uniform quality, and 

 in sufficient quantity to be used extensively. The gritstones of the 

 carboniferous system are the best stones that can be used for fire-proof 

 buildings. Gt'anites of all kinds belong to the class of m(?taphoric 

 rocks. They were certainly not deposited as they now are, nor were 

 they brought to tlieir present condition by any action of mere heat. 

 Originally, perhaps, of mechanical origin, they have been rendered 

 crystalline by long-continued chemical action, assisted by intense 

 heat and enormous pressure, while in the interior of the earth. In 

 this way have been produced not only the peculiar state of the mass 

 of the rock, but the fissures, and their varied contents, the veins of 

 metalliferous, and other matter that penetrate them, and all the 

 changes they induce in other rocks adjacent. Rocks of this kind are 

 double silicates. Slates., once thought. to be crystalline rocks, have 

 been produced from clay by mechanical pressure. Tlie fissile cliai-ac- 

 ter can be given to wax, or to any other substance whose particles are 

 minute, and all the varieties of color and texture, all degrees of fissile 

 nature, all the peculiarities of hardness and resistance may be 

 distinctly traced to the mechanical cause of the .phenomenon. It 

 requires however a careful examination of slates in the quarry, and 

 some habit of examining slate quarries, to aj)preciate the fact that 

 tliey are so nearly allied to clay pits, and that so narrow a line 

 separates materials apparently so very different. Nothing will be 

 _ said here of shales, which are transition rocks between clay and slate, 

 of the limestone flags, such as Stonefield slate, nor of such paving 

 stone as the Yorkshire flags. These hardly belong to architecture 

 though occasionally used in construction. Tliey are of a different 

 material, yet are formed in the manner of slates. 



