LITHOLOGY. 22/ 



blende and pyroxene, which were made with the idea of indicating why 

 hornblendic rocks are so predominant among the stratified schists in 

 New Hampshire (see p. 65). Among these analyses one will be found 

 which indicates the composition of the hornblende from the eruptive 

 diorite at Dixville notch. A comparison of this with the pargasite will 

 give the essential chemical differences between the hornblende of our 

 two kinds of diorite. 



Some of the metamorphic diorites are free from quartz, and some in- 

 clude quartz in their composition. On this basis, they are sub-divided 

 into plagioclase diorites, which are compounds of plagioclase and horn- 

 blende, with certain accessories, and quartz diorites, which contain quartz 

 in addition, but not in preponderating amount. 



Plagioclase Diorite. These rocks are light green in color, and are 

 granular and massive. Sometimes they form thick beds, and sometimes 

 only thin strata among other schistose rocks. They are much less nu- 

 merous than the quartzose varieties, and in composition they are very 

 complex. In a specimen from Pittsburg the hornblende is in such large 

 grains that it can be macroscopically recognized. Thin sections show 

 that this hornblende is of a light green kind, in which the dichroism and 

 absorption are marked. As usual, the grains of hornblende are fibrous 

 in their nature, and are all fringed out on the edges. In some grains 

 which are cut at right angles to the prism, the characteristic cleavage is 

 very plain ; but other grains appear to be made of a mere mass of needles, 

 and the fibrous structure prevents any such character as cleavage from 

 being recognized. I have represented a section of this rock in Fig. 3 on 

 PI. 12, but have introduced only the most general features that character- 

 ize it, and have left out the rare or accidental constituents. The mass of 

 the white portion of the rock is a triclinic feldspar, which is characterized 

 by the wide bandings that polarized light develops. These bands are 

 often indistinct, on account of the decomposition to which the feldspar is 

 very subject. As a result of this decomposition, epidote is a never-failing 

 constituent of the rock. This epidote is light yellow, somewhat dichroic, 

 and gives most brilliant colors in polarized light. By its abundance, and 

 by the presence of multitudes of little hornblende crystals, and of other 

 small minerals, the bands of color in the feldspar are sometimes nearly 

 obscured. Peculiar forms resulting from the decomposition of titanic iron 



