40 H. B. KUMMEL 



3. Metamorphosed arkose sandstone, distinguished by the 

 formation of a fibrous green hornblende. 



4. Lime-silicate hornfels (kalksilikat hornfelse). 



The two first groups differ only in the presence or absence 

 of tourmaline. They are very dense rocks, with a splinter-like 

 cleavage and abound in biotite. Traces of the original stratifica- 

 tion are preserved in the alternation of layers containing varying 

 amounts of mica. The tourmaline always appears as a secondary 

 mineral, in well-bounded black prisms up to three millimeters in 

 length and one in width. They are without definite arrangement, 

 the longitudinal axis being oblique to the stratification plane as 

 frequently as it is parallel to it. Each of the tourmaline crystals 

 is surrounded by a bright halo about half a millimeter in width, 

 caused by the absence of biotite. This may be accounted for 

 on the assumption that the iron and magnesia were consumed 

 in the formation of the tourmaline. The biotite crystals have 

 their tabular planes arranged parallel to the stratification planes. 



Feldspar is the chief constituent of the tourmaline-bearing 

 hornfels, and quartz is entirely wanting — a fact which indicates 

 that the original sediment was very deficient in silica, but 

 abounded in clayey materials. 



From such rocks, presenting clearly a crystalline structure, a 

 transition may be found to very dense masses in which, even 

 when highly magnified, no constituent parts, save biotite, can be 

 recognized. 



The lime-silicate hornfels is bright gray to green-gray in 

 color, dense and hard, and discloses, under the microscope, an 

 irregular aggregate of very small grains, with strong double 

 refraction, whose nature can be determined only from the larger 

 grains. The minerals common to rocks of this variety occur ; 

 a colorless pyroxene, closely related to diopside ; green horn- 

 blende ; colorless tremolite in fibrous and radiating aggregates ; 

 garnet ; vesuvian ; epidote ; while feldspar occurs commonly in 

 diminished quantity. This rock frequently exhibits an alterna- 

 tion of bright and dark layers, in the former of which diopside 

 usually prevails ; in the latter green hornblende and biotite. 



