104 REPORT UNITED STATES GEOLOGICAL SURVEY. 



furnish strong evidence in favor of the one or other assumption, the 

 number of variations, lithologically and structurally, are sufficiently- 

 great to produce a certain amount of confusion. As definite and usually 

 well-determined features of metaniorphic rocks, we may regard petro- 

 grapbic character in general, recurrence of certain lithological charac- 

 teristics in the same horizon, partial or complete stratification, and the 

 occurrence of certain minerals within definitely located vertical limits. 

 In several instances localities may be observed where the transition 

 from undoubted sedimentary into metamorphic beds was evident. This, 

 however, must be considered as an exception rather than the rule. 

 Within the districts I have examined; a few cases of tUJs kind were 

 found, and aided materially in interi3reting analogous, though not such 

 unmistakable occurrences. 



Gneiss may be regarded as the oldest metamorphic rock in the dis- 

 tricts examined. Its relative normal position to granite was estab- 

 lished at several points. A number of varieties occur, due in part to 

 the texture of the rock, due in part to inclosed accessory minerals. 

 Among these garnets, hornblende, and crystals of feldspar are the most 

 prominent. A diminution or total absence of this latter mineral, which 

 is part of the typical gneiss, causes it to change into micaceous schist. 

 Accessions of hornblende, or chlorite, frequently entirely supplanting 

 the mica, transform the rocks into hornblendic and chloritic schists. 

 Similar to an arrangement observable in the granites, such changes often 

 remain very constant in the same horizon, in proof of the metamorphic 

 character of the lithological group. The older sedimentary formations 

 have, in many instances, furnished material tor the formation of gneisses 

 and allied rocks. From the chemical composition of these we can draw, 

 to a certain extent, inferences as to the former unchanged condition of 

 the beds to which they owe their present existence. An incomplete 

 knowledge of the exact powers of the metamorphosing agents, as well 

 as of the manner in which such x)owers manifest themselves, precludes 

 any argument destined to establish, with certainty, the nature of the 

 rocks before their metamorphosis. 



Schists. — Micaceous, hornblendic, and chloritic schists occur, as such, 

 associated with other metamorphic rocks. Frequently they are due to 

 substitution of minerals within the gneiss, but they also are found 

 totally independent thereof. As metamorphics they are the result of 

 changed rocks that in their original condition showed a different chemi- 

 cal composition and physical structure from those furnishing the gran- 

 ites. If a suggestion may be offered, which, however, cannot at pres- 

 ent be proved, I would say^that argillaceous sandstones form granite. 

 With the decrease or increase of argillaceous matter in the sandstone, 

 the quantity of feldspar in the granite stands in direct proportion. 

 Silicious sandstones form quartzites. Shales, arenaceous in part, are 

 changed into gneisses, and if the quartz in them is predominant they 

 turn into schists. Admixtures of ferric oxygen compounds may result 

 in the formation of hornblendic and chloritic minerals, provided, in the 

 latter case, that magnesia be present. These deductions suggest them- 

 selves from the observations made in the Quartzite Mountains, more 

 jjarticularly. 



Quartzite. — Although generally quartzite cannot be classed with the 

 metamorphic, but metamorphosed rocks, we have found an instance 

 where it undoubtedly belongs to the former group. In the Quartzite 

 Mountains, enormous quantitie's of this material form high mountains. 

 A very complete alteration of the original sandstone has taken place, so 

 that now the single fragments resemble very closely pure quartz. 



