REPORT OF TEE CHIEF ASTRONOMER 489 



SESSIONAL PAPER No. 25a 



In order to make the relations of these fossiliferous horizons clearer, the 

 diagram of Figure 34 has been prepared. 



Evidently much more work needs to be done on this great monocline, but 

 it seems already probable that much if not all of the recognized Shasta- Chico 

 series of California and Oregon is here represented. The southern geosynclinals 

 of this age rival the one of the Hozomeen range in the almost incredible amount 

 of sedimentation which is manifested.* 



Pasayten Volcanic Formation. 



This formation has already been referred to as member A of the Pasayten 

 series, occurring at the very base of the Cretaceous series. It occurs in only 

 one part of the Boundary belt, on the densely thicketed slopes of the Pasayten 

 river valley. The exposures are not numerous but those observed were found 

 near the bottom and top as well as in the middle of the formation. At nearly 

 all of the outcrops the mass is composed of typical andesitic breccia. One large 

 outcrop near the Boundary slash showed a compact phase which may represent 

 a thick flow of somewhat vesicular andesite; this phase could not be followed 

 any notable distance through the brush. Elsewhere the breccia is clearly 

 dominant, so that it seems safe to describe the formation as essentially a breccia 

 of rather uniform composition. 



The breccia is extremely massive; at none of the outcrops was it possible 

 to find undoubted evidences of stratification. The estimated thickness — 1,400 

 feet — has been deduced on the assumption that the breccia conformably under- 

 lies the sandstone of member B of the Pasayten series ; this assumption, seems 

 quite justified by the fact of parallelism between the outcropping bands of the 

 two members. 



The volcanic mass consists very simply of angular blocks of porphyritic 

 andesite, cemented by a well consolidated ash. The blocks are of all sizes up 

 to those 12 to 15 inches in diameter. At one point the smaller fragments were 

 seen to be rounded as if water-worn. In general, however, evidences of sorting 

 or rounding by water-action were entirely absent. No other material than 

 andesite composes the fragments. The breccia was nowhere seen to be rendered 

 schistose through pressure. 



The blocks of the agglomeratic mass are dark greenish or brownish-gray. 

 The phenocrysts of altered pyroxene and plagioclase and occasionally of fresher 

 hornblende were usually conspicuous in the blocks, especially the larger ones. 

 Under the microscope the feldspar phenocrysts proved to average labradorite, 

 near Ab x Ar^. Some are zoned, with more basic labradorite in the cores and 

 basic oligoclase in the outer rims. The pyroxene seems to be a common green 

 augite; it is generally pretty thoroughly altered to uralite and chlorite. The 

 hornblende is a common green variety; it is not so abundant as the augite. 



* See J. S. Diller and T. W. Stanton on ' The Shasta-Chico Series ' ; Bull. Geol. 

 Soc. America, Vol. 5, 1894, pp. 435-464-. 



