COMPOSITION OF CERTAIN WEATHERED ROCKS. 



409 



a. Shale, altered considerably, light-brown and friable. Carbonates in 

 small quantity form a part of the mass of the rock. This shale is Neo" 

 comian. 



h. Serpentine, very much weathered, interwoven with seams of quartz and 

 calcite; darker colored than a; fibrous tendency slight. 



c. Limestone, weathered, very friable, light slate-colored. 



d. Calcareous shale, next to the same serpentine dike mentioned in con- 

 nection with series III. It is hard, compact, dark-colored ; it contains 

 seams of calcite, but the analysis was made on material from which this 

 secondary carbonate had been almost completely removed. In the quartz 

 seams a zeolite (natrolite) occurs, and sparingly distributed small particles of 

 pyrite are to be found. The shale is very much altered, particularly at the 

 contact. 



e. A yellowish-green talc-like rock, forming a part of the serpentine dike 

 at this point and containing considerable chromite. It has a resinous luster, 

 and a tendency to fibrous structure. Limestone intervenes between this 

 rock and the calcareous shale d. 



Specimens c, d, and e properly belong to a separate series, of which c and 

 e are extremes. They are grouped here for convenience. 



Series IV — Analyses of Specimens from near Arroyo del Cerro. 



(•233) (235) (236) (238) (239) 



H^O at 100° C I 



H,0 above 100° C j 



SiOa 



CO2 



PA 



Cr203 



AlA 



FeA 



Feb 



NiO 



MnO 



MgO 



CaO 



Na20 



K2O 



a 



b 



c 



d 



e 



8.74 j 



3.32 



0.76 



1.41 



0.44 



12.51 



2.33 



6.24 



12.40 



45.64 



41.52 



21.19 



44.56 



32.27 



4.59 







26.84 



17.62 







0.27 







2.55 



0.16 



trace 



0.12 















5.19 



15.42 



1.57 



0.39 



3.12 



11.45 



3.40 



3.50 



1.52 



1.27 



trace 



3.7? 



1.07 



undet. 



5.21 



5.05 



















0.19 



0.33 



0.29 



3.61 



trace 



trace 



4.62 



36.84 



1.39 



3.39 



33.30 



8.11 



0.44 



35.61 



12.70 



0.41 



3.13 







undet. 



3.09 



trace 



1.86 



---- 



undet. 



0.88 



trace 



99.96 



101.06 



96.19 



99.65 



100.70 



Series IV shows a contact of shale a with serpentine h, the analyses of 

 which are given in the above table. A transition is also given of limestone c, 



