REPORT OF THE CHIEF ASTRONOMER 781 



SESSIONAL PAPER No. 25a 



voluminous andesites from basalt by differentiation witbin volcanic vents, thus 

 increasing tbe known area wbere basaltic magma bas been erupted. 



1. Only one large field of fissure eruption is certainly represented in tbe 

 Boundary belt, namely, that of tbe probably-Cambrian Purcell Lava. Tbougb 

 this rock is everywhere profoundly altered, its composition, throughout thousands 

 of square miles, is basaltic, with a tendency in places to the (augite) andesitic. 

 A very thin and quite local flow of liparitic lava is closely associated with tbe 

 basaltic type at one point in the Purcell range. It is possibly the result of 

 assimilation of acid rocks by the basalt, but its existence in no wise affects the 

 statement that this fissure-eruption is essentially uniform and basaltic in com- 

 position. Nearly all of Paleozoic time, all of Mesozoic time, and much of Ter- 

 tiary time elapsed before the vast Columbia lava-field was completed a short 

 distance south of tbe Boundary Line. In that later and greater flood the lava 

 varies from common olivine basalt to the more andesitic facies represented in 

 the porphyritic, olivine-free phase of the Purcell Lava.* Calkins has pointed 

 out the resemblance between this ancient porphyritic lava and an equally 

 remarkable Miocene lava in Washington. The constancy of the type through 

 so long a period is thus shown in details of structure as well as in the manner 

 of its extrusion. 



2. Basaltic magma is the only one known to have crystallized as visible 

 bodies in each of the ranges between the Great Plains and the Pacific. With 

 two exceptions, either tbe Purcell Lava or its approximate chemical equivalent, 

 gabbro (rarely passing into diorite), form the only igneous masses seen where 

 the Boundary belt crosses the Lewis, Clarke, Galton, MacDonald, McGillivray, 

 Yahk and Moyie ranges. The first exception referred to is the liparite flow just 

 mentioned. Tbe other is the secondary granite of the Moyie sills. West of the 

 Purcell Trench great abyssal injections (batholiths and stocks) first appear in the 

 Boundary section, and it is west of the Purcell Trench that strong petrographic 

 ▼ariety appears. The primary basalt never failed to be erupted in any of the 

 ranges west of the trench, but many of its abyssal injections were so large as to 

 furnish heat sufficient for much assimilation with consequent differentiation of 

 non-basaltic rock bodies. 



3. The many reappearances of basaltic magma as lavas or as injected masses 

 is illustrated in the following chronological table, which embodies a partial 

 list of the basic volcanic formations recognized by Dawson, G. O. Smith, and 

 others, in regions close to the Forty-ninth Parallel. 



• See F. C. Calkins, Bull. 384, U.S. Geol. Survey, 1909, p. 51. 



