THE NONMETALLIC MINERALS. 333 



as a rule much less fusible than are the glacial or stratified clays, and 

 are used mainly in the manufacture of fire brick, sewer pipe, terra 

 cotta stoneware, as crocks, fruit jars, jugs, etc., glass and gas retorts, 

 smelting pots, etc. Some of these articles are made direct from the 

 natural clays, while others are from a mixture of several clays or of a 

 clay mixed with powdered quartz and feldspar. 



For ordinary brick-making purposes a great variety of materials 

 are employed; in some cases residuary deposits, and in others alluvial 

 and sedimentary. Throughout the glacial regions of the United States 

 a fine unctious blue-gray material, laid down in estuaries during the 

 Champlain epoch, the so-called Leda clays, are the main materials used 

 for this purpose. Such are also sometimes used in making the cheaper 

 kinds of pottery. The bowlder clays of the glacial regions are also 

 sometimes used when sufficiently homogeneous. 



The prevailing colors of the Leda clays are blue -gray or yellowish. 

 They all carry varying amounts of iron, lime, magnesia, and the 

 alkalies, and when burned turn to red of varying tints. They fuse 

 with comparative ease and are used mainly for brick and tile making 

 and for the coarser forms of earthenware, such as flower pots, being 

 as a rule mixed with siliceous sand to counteract shrinkage. The 

 mining of such material is of the simplest kind, and consists merely of 

 scraping away the overlying soil and sand, if such there be, and remov- 

 ing the clay in the form of sidehill cuts or open pits. 



Plate 18, facing this page, shows a cut in one of the beds at Lewiston, 

 Maine. The material here is fine and homogeneous, of a blue-gray 

 color, and contains no appreciable grit. It is mixed with siliceous 

 sand and used for making bricks, baking red. An analysis of the 

 material in its air-dry state yielded results as below: 



Silica (SiO 2 ) 56. 17 



Alumina ( A1 2 O 3 ) 24. 25 



Ferrous oxide (FeO) 3. 54 



Lime(CaO) 2.09 



Magnesia (MgO) 2.57 



Potash(K 2 O) 4.06 



Soda (Na,O) 2.25 



Ignition (H,O) 4.69 



99.62 



Under the microscope these clays are seen to be made up of beauti- 

 fully fresh, angular bits of quartz, feldspar, mica, hornblende, and 

 augite, with more rarely tourmalines, zircons, and other refractory 

 minerals, with a basis of extremely fine undetermined material which 

 may perhaps be kaolin, though the general structure of the clay is 

 such as to suggest it owes its origin mainly to mechanical trituration, 

 rather than chemical decomposition. The appearance of the Lewiston 

 clay under the microscope is shown in Plate 16, fig. 1. (See Specimens 



