August 10, 1906.] 



SCIENCE. 



177 



east of Pikes Peak, discussing the Bouteloua 

 formation as to its structure. Aven Nelson 

 publishes 15 new species of seed plants from 

 the Eocky Mountain region. George J. Peirce 

 gives an account of Anthoceros and its Nostoc 

 colonies, showing the fallacy of Prantl's argu- 

 ment that because cavities and hairs do not 

 develop in the usual way except where the 

 colonies are, the liverwort must profit by such 

 an association. 



DISCUSSION AND CORRESPONDENCE. 



GLACIATION IN THE SONORAN PROVINCE. 



To THE Editor of Science: In the current 

 number of Science^ Dr. Frederick J. H. Mer- 

 rill directs attention to moraine-like accumu- 

 lations of debris observed by him at a number 

 of localities in northern Sonora and southern 

 Arizona and New Mexico; of which accumu- 

 lations part are well known to me — though my 

 provisional interpretations differed from those 

 of Dr. Merrill. The district is too extensive 

 and too little known to warrant broad gen- 

 eralizations or to justify negation of the sug- 

 gestions offered by so acute an observer as 

 Dr. Merrill ; yet future observers may be aided 

 by the alternative suggestions growing out of 

 my own observations in half a dozen journeys 

 in the saddle (with others by rail and stage) 

 through the Sonoran province. 



So far as my observations go, the more ex- 

 tensive debris accumulations of the type de- 

 scribed by Dr. Merrill (including those in the 

 borrow-pit near Nogales and thence south- 

 ward to Imuris) are confined to districts of 

 late Mesozoic or Cenozoic vulcanism; while 

 in some cases the accumulations appear to 

 pass both horizontally and vertically upward 

 into undoubted volcanics, much as the ttiff 

 beds underlying each table mountain in Cali- 

 fornia grade into more firmly lithified lava 

 sheets^ Concordantly, my interpretation of 

 the debris accumulations was that they were 

 originally, and sometimes are now, volcanic 

 tuffs and breccias much like those found fur- 

 ther northward along the Pacific coast save 

 that the brecciated structure is more striking 

 than I have seen elsewhere. Frequently the 

 breccias and lava sheets overlie massive blue 



M^olume XXIV., pp. 116-118. 



limestones undoubtedly equivalent to the vast 

 Mesozoic (Cretaceous and Jurassic) limestone 

 series of northeastern Mexico, and probably 

 equivalent to the shale-mixed limestones of 

 the Sierras, especially in southern California; 

 and within a few miles of the borrow-pit south 

 of Nogales (by which I once had occasion to 

 camp for three days while awaiting official 

 documents from Mexico) there are consider- 

 able exposures of this limestone, weathered 

 as usual into the peculiar rugose surface — 

 miniature mountain ranges and valleys — 

 found by Hill and others to be characteristic 

 of the Mesozoic limestones in the eastern 

 Sierra Madre. The calcareous deposits are 

 seen in places to rest (and probably every- 

 where lie) unconformably on granites, which 

 in western Sonora and southwestern Arizona 

 are of wide extent; yet neither on the lime- 

 stone nor on the granitic terranes have I ob- 

 served such debris accumulations as those so 

 characteristic of the Imuris or Opodepe valley. 

 Certain of the accumulations in this valley 

 seemed to me well worthy of critical study as 

 breccias primarily volcanic though accumu- 

 lated in part by concurrent aqueous agencies, 

 and recording in themselves a peculiarly com- 

 plex volcanic history; for in several sections 

 there are imbedded in the clay-like matrix 

 angular boulders yards in dimensions, some- 

 times containing included boulder-like masses 

 feet across, themselves made up of brecciated 

 constituents inches or less in diameter. A 

 part of the heterogeneous accumulations both 

 in Imuris Valley and elsewhere were inter- 

 preted by me as rearranged breccias trans- 

 ported to limited distances and redeposited by 

 sheetfloods or other freshets during the rather 

 remote periods in which the lava sheets and 

 tuff beds were newly exposed to surface ero- 

 sion. In this connection it is to be noted that 

 the province in question is the type region for 

 sheetflooding, and that both the lower slopes 

 of the sierras and the intervening plains are 

 shaped by this agency; and also that sheet- 

 flood deposits are normally heterogeneous and 

 ill-assorted accumulations of coarse and fine 

 material, seldom perceptibly stratified or 

 graded from coarse to fine either horizontally 

 or vertically in the single section. 



