REVIEWS 545 



The gravels of the Lafayette are rather extensively used as road metal and 

 railroad ballast. 



Hillebrand, W. F. Some Principles and Methods of Rock Ana/ysis. Bull. 

 176, U. S. Geol. Surv. 8vo. Pp. 115, Figs. 15, 1900. 

 Detailed discussion of the methods to be followed in analyses of the silicate rocks. 

 Stress is laid on the importance of complete and thorough analyses, and the chemist 

 is warned against neglect to determine elements (such as strontium, barium, vanadium, 

 etc.), often disregarded as unimportant. 



Hopkins, T. C. Clays and Clay Industries of Pennsylvania. Ill, Clays of 

 the Great Valley and South Mountain Areas. Appendix to the Ann. 

 Rept. Pa. State College for 1 889-1 goo. 8vo, pp. 45, igoo. 



Over half the paper is taken up with a discussion of the white clays of the Great 

 Valley; the remainder containing short chapters on respectively the red brick, paving 

 brick, and tile industries ; with a brief account of the economic products, other than 

 clays, of southeastern Pennsylvania. 



The most valuable clay deposits in southeastern Pennsylvania are those occurring 

 in the Cambro-Silurian areas of the Great Valley and South Mountains. These clays 

 are typically white ; commonly bluish-white to gray on fresh exposures, but soon weath- 

 ering pure white. Closely associated with the white clays are others stained more or 

 less by iron oxide, and being therefore yellow to brown in color. The white clays are 

 invariably high in silica, and low in alumina. Their iron content is also low, but the 

 alkalies are always too high to give refractory material. Genetically, the white clays 

 are direct decomposition products of light-colored hydromica slates which occur 

 interbedded with other Cambro-Ordivician rocks. In certain openings the clays are 

 shown grading into the undecomposed slates. They were originally intercalated beds 

 in a series of quartzites, limestones, etc.; but on the weathering of the rocks the clay 

 derived from the disintegrated slates crept down hillsides, filled cavities in the other 

 rocks, etc. — so that now the deposits are somewhat irregular, lenticular masses of 

 varying extent. At present the clays are largely used in the manufacture of paper, 

 and to a less extent for white tiles, and white and enameled brick. 



Red building brick is made, in the Great Valley, from residual limestone clays, 

 residual shale clays, and alluvial deposits. Of these classes the first is the most 

 important. The clays residual from limestone make good brick, though often carrying 

 numerous fragments of limestone, quartz, etc. 



Vitrified brick is manufactured at six plants. Three of these use Triassic shale ; 

 one, Hudson River shale ; while the remaining two use shales from the Cambro-Ordo- 

 vician series. 



Ornamental brick and tile are manufactured at several plants, various materials 

 being in use, including clays, slates, etc. 



Ihlseng, Magnus C. The Road- Making Materials of Pennsylvania. Bul- 

 letin 6g, Pa. Dept. Agriculture. Pp. 104, Figs. 1-17, relief map, col- 

 ored geologic map, igoo. 

 General discussion of road materials, their qualities, and testing; influence of 

 topography on road construction ; methods of constructing and repairing roads, etc. 



