APPENDIX TO REPORT OF TflE SECRETARY. 81 



might be represented at one end bj a coarsely granular micaceous rock 

 composed chiefly of orthoclase, and is sometimes hornblendic and some- 

 times augitic, at the other end a highly porphyritic trachyte consisting 

 of well-developed orthoclase feldspar imbedded in a fine paste highly 

 charged with peroxide of iron. Between these masses of trachyte are 

 intercalated, though in subordinate quantities, beds of dolerite showing 

 distinct crystals of striated feldspar with great abundance ofaugite and 

 magnetite. In the earlier and grander periods of the eruptions the fol- 

 lowing sequences may therefore be recognized : first, propylite; second, 

 andesite; and, third, interblendcd trachyte and dolerite. Still later than 

 these, and occurring at new centers of eruption, were outpours of rhyo- 

 lite, while last of all were erupted around the outskirts of the district 

 great quantities of true basalt. There does not appear to be any single 

 locality where all the groups of rocks are found superposed ; neverthe- 

 less the relative ages do not admit of any doubt, whether the various 

 beds are superposed or not; but while furnishing a general verification 

 of the sequence, the district of the High Plateaus presents the fact with 

 certain modifications which may be set forth in the following manner : 

 In the lithological scale propylite and hornblendic andesite are very 

 nearly intermediate between the extremes of acid rocks represented by 

 rhyolite, and basic rocks represented by basalt. Taking propylite as a 

 starting-point in the scale of classification, we find two divergent series 

 proceeding on the one hand toward the acid end of the scale, and on the 

 other toward the basic end. As we follow the eruptions down into the 

 later epochs, we find that both series are represented in a certain sense in- 

 dependently of each other so that they intercalate; the acid becoming at 

 one end more acid with the progress of the volcanic cycle, and the basic 

 rocks becoming more basic. Each series seems to pursue its own order 

 and to be subject to its own law, so that, being originally divergent, they 

 become more and more widely separated in their lithological characters 

 as the cycle proceeds. Thus at the commencement of the activity we 

 have propylite and hornblendic andesite, which are closely assimilated 

 to each other in their physical characteristics; at the middle stage we 

 have trachytes and moderately basic dolerites, which are moderately 

 separated, and at the close we have rhyolites and basalts, which stand 

 at the opposite ends of the scale. 



Planofpuhlication. — In the geological branch of the work the plan has 

 been adopted as far as possible of publishing monographs, each embrac- 

 ing all the studies made by the corps of any particular region to which 

 it relates, preferring this to a system of annual reports consisting of 

 resumh of the field-notes of each season. In the preparation of these 

 monographs, relief maps or plates are constructed on a scale of two miles 

 to the inch or larger, vertical and horizontal scales being equal; and to 

 correspond with each relief map a stereogram in plaster is constructed 

 on the same scale, designed to exhibit such a surface as would appear 

 had there been no degradation by atmospheric agencies, but displace- 

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