548 REPORT OF NATIONAL MUSEUM, 1890. 



The term syenite-gneiss is used to designate a rock of the composition of 

 syenite, but with a gneissoid structure. The name granulite is applied 

 to a banded quartz-feldspar rock the constituents of which occur in the 

 form of small grains and show under the microscope a mosaic structure. 

 The Saxon granulites shown (specimens numbered 36126, 36658 to 36668, 

 etc.) are regarded by Lehman as eruptive rocks altered by pressure. 

 Halleflhita is a Swedish name for a rock resembling in most respects the 

 eruptive felsites or quartz porphyries already described. Such, how- 

 ever, show a banded structure and are as a rule regarded as meta- 

 morphic rocks (see specimens 35673, 36676, 36677, and 38459 from 

 Dauuemora and Kopparberg, Sweden). Porphyroid is also a felsitic 

 rock with a more or less schistose structure and with porphyritic feld- 

 spar or quartzes (specimens 36721, 36722, and 36723 from the Ar- 

 dennes, France). 



2. The crystaline schists. 



Under this head are grouped a large and extremely variable class of 

 rocks differing from the gueisses mainly in the lack of feldspar as an 

 essential constituent. They consist therefore essentially of granular 

 quartz, with one or more minerals of the mica, chlorite, talc, ainphibole, 

 or pyroxene group. In accessory minerals the schists are particularly 

 rich. The more common of these are feldspar, garnet (specimen 36112), 

 cyanite, staurolite (specimen 36764), tourmaline (specimen 28574), epid- 

 ote, rutile, magnetite, menacaunite, and pyrite. Through an increase 

 in the j>roportional amount of feldspar the schists pass into the gneisses 

 and through a decrease in mica, hornblende, or whatever may be the 

 characterizing mineral, into the quartz schists in which quartz alone is 

 the essential constituent. Occasional forms are met with quite lacking 

 in quartz and other accessory minerals and consisting only of schitose 

 aggregates of minerals of a single species, as is the case with the pyro- 

 phyllite schists (or more properly schistose pyrophyllites) from North 

 Carolina (specimen 27665), talcose schists from Michigan (specimen 

 35799) and St. Lawrence County, New York (specimen 36131), and with 

 the more massive " soapstones " from Maryland (specimen 27331) and 

 Vermont (specimen 25288). 



The rocks of this group are characterized as a whole by a pronounced 

 schistose structure, due to the parallel arrangement of the various 

 constituents, this structure being most pronounced in those varieties in 

 which mica is the predominating mineral. They are ordinarily consid- 

 ered as having originated from the crystallization of sediments, and in 

 many cases the microscope still reveals existing " traces of the original 

 grains of quartz sand and other sedimentary particles of which the 

 rocks at first consisted." Like the gneisses they are in part, however, 

 mechanically deformed massive rocks and their schistosity in no way 

 relates to true bedding. 



The varietal names given are dependent mainly upon the character 

 of the prevailing ferro-magnesiau silicate. We thus have mica-schists, 



