calkins.] Petrography of the John Day Basin 



159 



earlier How and prevented from escaping immediately, when the 

 lava was poured out by viscosity; of the acid magma. The 

 presence of superheated vapor is well known to be a potent aid 

 to crystallization, and is supposed to have here determined the 

 formation of the feldspar fibres. In the later now the water is 

 presumed to have been present in too small quantity to produce 

 such an effect. 



THE MIOCENE BASALTS. 



Field Characters and Classification. — The great basalt series 

 above the John Day is mainly built up of heavy lava flows, the 

 iuterbedded tuff s being relatively of insignificant volume. These 

 tuffs, as far as observed by Dr. Merriam or the writer, are also 

 basaltic. Penetrating the John Day beds at several localities 

 and connected with the overlying lavas are numerous basalt 

 dykes, whose occurrence, combined with the predominance of 

 massive lavas over tuffs, seems to give evidence that the pre- 

 vailing mode of extravasation was by quiet up-welling from 

 fissures rather than by explosive eruption from craters. 



The structural features of the basalts as observed in the field 

 are not especially remarkable. The development of columnar 

 structure in both dykes and flows is general, the size and per- 

 fection of the columns increasing with the thickness of the mass 

 in which they occur. The upper portion of the flows have the 

 usual vesicular character, and the slaggy and ropy surfaces 

 have been especially well xueserved in certain cases when the 

 overlying layer has been of tuff rather than of lava. 



Laboratory investigation has shown that the mineralogical 

 constitution of these "basalts is remarkably constant . They are 

 without exception normal olivine basalts of probably the most 

 common type. This uniformity of character is not confined to 

 the limited region discussed in the present paper, but holds good 

 for specimens collected by the writer at various points in Northern 

 Oregon and Central Washington*. 



The basic lavas occurring in small volume interstratified with 

 the lowest beds of the Mascall formation are of another type, 



* Through the courtesy of Dr. George Otis Smith of the United States Geologi- 

 cal Survey, the writer has been enabled to examine sections, prepared in the Sur- 

 vey's, laboratory, of basalts collected in the.Ellensburg, Washington, quadrangle. 



