ROOFING SLATE OF IGNEOUS ORIGIN 



21 



elusive. Dr. Cross states that the specimens are composed 

 largely of feldspar, calcite, chlorite, and some other minerals. 

 All are evidently greatly sheared. They present, however, no 

 direct evidence of derivation from the kind of rock represented 

 by the specimen from the dike near the tunnel entrance. They 

 contain no augite or plagioclase, as such, and show no transition 

 in texture. While it is quite possible that these rocks may have 

 been derived from an igneous rock like the gabbro, such deriva- 

 tion is not shown by microscopic examination of the specimens 

 submitted. 



Chemical investigation. — A typical specimen of the green 

 slate, from near the middle of the band, was selected for partial 

 analysis in the laboratory of the U. S. Geological Survey, and 

 the results of this analysis are presented below, as No. i. The 

 second analysis given below was quoted to the writer by Mr. C. 

 H. Dunton, manager of the Eureka Slate Co., but the name of 

 the analyst was unknown to him. The two analyses agree 

 suflficiently closely, and are probably fairly representative of the 

 chemical composition of the green slates. 



ANALYSES OF THE "GREEN SLATES " FROM SLATINGTON, CALIF. 



Silica (SiOg) 



Alumina (AI3O3) and titanic oxide (TiOg) 



Iron oxides (FeO, FcgOg) 



Lime (CaO) 



Magnesia (MgO) 



Akalies (Na^O, KgO) 



Carbon dioxide (COg) 

 Water 



47-30 

 15-53 

 8.00 

 7.83 

 7.86 

 3-17 

 9.92 



1. By W. T. Schaller, U. S. Geological Survey laboratory. 



2. Analyst unknown. Analysis quoted by Eureka Slate Co. 



It will be seen that these analyses differ widely from those of 

 any normal clay slate, and even if no structural evidence were 

 at hand, the chemical composition of the green slate would be 

 sufificient to suggest that their origin was probably from igneous, 

 not sedimentary, rocks. 



For the purpose of indicating the class of rocks from which 



