26 



EDWIN C. ECKEL 



has no very wide range. This is due to the fact that in the Atlantic 

 states, which, as above noted, furnish the bulk of the analyses, 

 the true roofing slates are practically confined to the formations 

 below the Medina. Two exceptional instances are represented 

 in the series of analyses. One is a highly metamorphosed slate 

 of doubtfully Carboniferous age ; the other is a rather soft slate 

 of undoubted Upper Devonian age. With the exceptions of 

 these two eastern slates, and of the Jurassic slates of California, 

 the analyses represent merely the Ordovician, Cambrian, and, 

 doubtfully, pre-Cambrian. The table below gives the geologic 

 distribution of the analvses in detail : 



■Age 



Jurassic ------ 



Carboniferous? . . - - 



Devonian ------ 



Upper Silurian . . - . 

 Ordovician - _ . - - 



Cambrian ----- 



Pre-Cambrian, Cambrian, or Ordovician 



Number of 

 Analyses 



I 

 I 

 I 

 O 



- I I 



i6 



- 6 



36 



TABLE I. 

 RESULTS OF THIRTY-SIX ANALYSES OF AMERICAN ROOFING SLATES. 



Silica (Si02) 



Alumina (AlgOg)-)- titanic oxide (TiOs) 



Ferrous oxide (FeO) 



Ferric oxide (FegOg) 



Total iron oxides (FeO + FcgOg) 



Lime (CaO) 



Magnesia (MgO) 



Potash (K2O) 



SodaCNagO) 



Total alkalies (KgO + NagO) 



Pyrite (FeSg) 



Carbon dioxide (COg) 



Water of combination 



Moisture, below 110° C 



Number of 

 Determina- 

 tions 



33 



32' 



19 



19 



33 



34 



34 



26 



26 



29 



18 



16 



15 

 16 



Maximum 



68.62 



24.71 



6.81 



7. 10 



10.66 



5-23 

 6.43 

 5-54 

 3-15 

 8.68 



Average 



60.64 



18.05 



3.66 



2.25 



I. 19 

 4-74 

 0.38 

 1.47 



3-51 

 0.62 



Minimum 



54-05 

 9-77 

 0.97 

 0.52 

 2.18 

 0.00 

 0. 12 

 0.72 

 0.20 



. 1-93 



' In nineteen of these, titanic oxide was separately determined, though it has 

 here been included with the alumina. The average of these nineteen determinations 

 gave 0.73 per cent. TiOg. 



