APPENDIX TO REPORT OF THE SECRETARY. 79 



long one. The individual flows are very numerous, and represent all 

 the great groups of eruptive rocks. In many cases the quantity of ma- 

 terial extravasated is so great that the eruptions may well be called 

 massive; not, however, of such marvelous extent as are asserted to 

 have been poured forth from fissures during the basaltic period in 

 Oregon and Northern California ; but there are many individual sheets 

 which surpass in magnitude any which is known to have emanated 

 during recent or modern times from any existing volcanic vent at a 

 single eruption. From what openings these masses were extravasated 

 it is usually difiBcult to fix with certainty and precision. So vast are 

 the accumulations and so expansive are the sheets, and, at the same 

 time, so numerous, that wheresoever they were emitted the earlier vents 

 must have been buried by later deluges of lava ; and even the more 

 recent vents, except in the case of the latest basalts, have been swept 

 away by slow erosion in the long period which has elapsed since their 

 activity was extinguished. There are, however, still remaining, distinct 

 traces of localization of eruptive activity in the form of greater accumu- 

 lations, at some points from which, in mcst directions, the total thick- 

 ness of the volcanic series appears to attenuate. Moreover, in those 

 central localities of maximum accumulation there appears to be a large 

 amount of what might be called, in a certain sense, unconformity of the 

 various eruptions, and greater irregularities in their bedding, as com- 

 pared with the more even layers and more regular distribution of the 

 sheets more remote from these centers. This fact appears to be of gen- 

 eral application also to existing volcanic regions of great extent. Cap- 

 tain Button has succeeded in locating at least five areas, from which 

 the various overflows appear to have emanated, and believes that further 

 research might result in the determination of others. 



At the time these eruptions were in progress it is probable that the 

 country was not an elevated one as at present, though it may have 

 been a rising area. The great displacements, consisting of faults 

 of extraordinary length and persistency, took place after the close or 

 during the decadence of the principal eruptions, and it was at this lat- 

 ter epoch that the greater part of the general elevation of this portion 

 of the plateau province occurred. During its progress many eruptions 

 must have taken place, and their later age is readily identifiable ; but 

 none of them were comparable in extent and in the volume of ejected 

 materials with older eruptions prior to the j;reat displacements. Al- 

 though it is ordinarily not difiicult to determine whether a particular 

 event preceded or followed some other within the locality, there seems 

 to be no way of correlating these different events strictly with the epochs 

 which are designated by the sedimentary formations of the adjoining 

 country, and it is therefore impossible to determine the exact period in 

 the chronological scale at which the faulting took place, further than 

 the fact that it must have occurred long enough after the close of the 

 eocene to allow for the accumulation of these vast bodies of volcanic 



