228 



MINERALOGY AND LITHOLOGY, 



are also characteristic of this rock. Gray masses filled with remnants of 

 the black iron oxide are scattered about, and in some cases the titanic 

 iron has entirely disappeared, leaving only the gray material. This sub- 

 stance is usually traversed by lighter lines, which being determined by 

 the cleavage or lamination of the original mineral, are regular in their 

 directions, and therefore symmetrical forms are produced which, in some 

 cases, are remarkably like fragments 'of organisms. One of these forms, 

 which is represented in Fig. 5 on PI. 2, has been already spoken of. 

 Titanic iron is a mineral which is not easily decomposed ; but its decay 

 is a characteristic feature of the diorites, and has resulted from a reaction 

 which has taken place between the lime, separated from the feldspar, 

 and the titanic acid (see p. 40). The iron has perhaps entered into the 

 composition of the chlorite, which is sparingly present. Some sections 

 of this rock contain grains of augite. This augite is of a light red color, 

 but I have not introduced it into the figure, for although occasionally 

 present in quite large grains, it is in no sense characteristic of the rock, 

 and is usually absent. 



A specimen from Littleton is much more compact, and is so fine in 

 texture as to appear like a homogeneous mass. In thin sections it is, 

 however, nearly the same as that last described, but is more decom- 

 posed, and hence contains more chlorite, epidote, and altered titanic 

 iron. Biotite and calcite are also present. 



These two specimens from Pittsburg and Littleton, both having a 

 specific gravity of 2.96, were analyzed with the following results : 



Littleton. 



45.56 



16.57 



•36 



9.40 



.20 



Silica, 



Alumina, . 



Iron sesquioxide, 



Iron protoxide, 



Manganese protoxide 



Lime, 



Magnesia, 



Potash, . 



Soda, 



Titanic acid, . 



Water, . 



Carbonic acid, 



Pittsburg. 

 48.79 

 16.97 

 1.69 

 8.97 

 .20 

 9.98 

 6.98 



3-30 

 1. 10 

 2.65 



8.01 

 10.34 

 1.20 

 2.55 

 1.20 

 3-93 



100.63 



100.34 



