48 FIELD MUSEUM OE NATURAL HISTORY 
Roofing slates are represented by specimens from various 
quarries in New York, Vermont, and Virginia. Specimens of 
slate from the Welsh quarries illustrate the manner in which 
blocks of slate are split or cleaved into a series of laminae or thin 
plates, which may be afterwards cut to uniform sizes and used 
for roofing. 
Hall 68. 
CLAYS, SANDS, AND SOILS. 
The arrangement of cases in this hall divides it into two 
alcoves on either side of a central passage. The space to the 
right upon entering from Hall 67 is occupied by a clay collection, 
the space to the left by the soils, sands, and cements. 
CLAYS. 
The key to the arrangement of the clays is given by a synop- 
tic collection which occupies the entire length of the front of the 
two cases facing the central passage. In this series are displayed 
typical specimens of each of the twenty-nine classes of clays of 
Orton’s classification. Their relations to each other and to the 
rocks from which they are derived are expressed by a method 
of grouping and by a number of lines connecting the various 
groups. If these lines and the associated specimens be consid- 
ered as a kind of genealogical tree of the clays the mieaning 
of this series will be evident. Each specimen of clay in this 
series has received in order a class number printed upon the label 
in Roman numerals. As a similar numeral is placed upon the 
labels of all other clays shown in this hall, the nature of any clay 
shown may be determined in a moment by referring to the speci- 
men in the synoptic collection with the same class number. 
After looking over the synoptic collection the visitor should in- 
spect the series illustrating the effects upon their useful proper- 
ties of the impurities commonly present in clays. The greater 
part of the space devoted to clays is occupied by specimens ar- 
ranged according to industrial values. These are arranged in 
three series, each subdivided and numbered according to the 
synoptic collection. Each class of specimens is accompanied 
by a descriptive label. Each specimen is accompanied by a 
carefully burned briquette of the same clay which illustrates 
the color, texture, shrinkage, vitrification and other properties 
of the clay in question when burned. The three collections are: 
