204 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



OCCURRENCE OF YOLCANIC ROCKS ON THE NORTH AMERICAN CON- 

 TINENT. 



The North American continent can appropriately be separated into 

 three sections for our purposes — into the eastern, middle, and western 

 divisions. In the first the volcanic occurrences are sporadic, limited as 

 to extent, and all or nearly all belonging to one period of relatively 

 recent age. In the second, the development of volcanic activity was 

 at its minimum. Very few occurrences belonging to that group of 

 geological history have taken place in that section, and wherever found 

 are but local, and of no importance either in the geological series or to- 

 pographical features of the region exhibiting them. Differing from both 

 of these, and presenting groups of the utmost interest and decided varia- 

 tion, is the western section. Beginning with the Alaskan peninsula, 

 we observe volcanics composing portions of the continent* and many of 

 the outlying islands. Throughout the mountainous chain, extending 

 through British Columbia southward toward the United States, volcanic 

 occurrences have been observed. Explorations of that mountain system 

 are wanting which would furnish us with suiJicientdata to make any gen- 

 eralizations whatever, or even to prevent those meager data obtainable 

 in a connected form. Within the area of the United States we are more 

 thoroughly acquainted with the occurrences of volcanic rock, and it is 

 to the writings of explorers like Hayden, King, Newberry, Eicbthofen, 

 and Whitney that we are indebted for our knowledge of their existence 

 and characteristics. 



Through the Western States and Territories the number of varieties 

 exhibited by volcanic eruptions not only, but also the enormous hori- 

 zontal extent of the groups, invite study and investigation. Owing to 

 the comparative completeness of the series, reaching from the Plutonic 

 eruptives to active or nearly active volcanoes of the present day, they 

 furnish a more complete record of the history of volcanic rocks than 

 any other region thus far known. In the heart of the Rocky Mountains 

 we meet with innumerable exposures of the older groups, which are re- 

 placed by younger ones as we proceed west and south toward the pres- 

 ent shorelines. In California there are basaltic groups, which, accord- 

 ing to Whitney, may almost be regarded as still active. In Utah and 

 Nevada, Gilbert has found regions that looked remarkably "fresh," and 

 Powell has discovered legends among the Indians referring to volcanic 

 eruptions during historical times. 



Southward, beyond the boundary of the United States, the volcanic 

 areas continue into Mexico and Central America. As we approach 

 their shores, we find that the formations (volcanic) are younger, until 

 still active volcanoes are found. This state of things continues on 

 through the Cordilleras to the southern end of South America. We 

 have, therefore, in the Eocky Mountains of the United States an area 

 of exhausted volcanic activity. Upon observation, it will be found that 

 the former volcanic outflows and eruptions took place along a line ap- 

 proximately corresponding with the present shore-line of the Pacific 

 Ocean. The arrangement of our mountain ranges is essentially a north 

 to south one, and associated therewith we see that the arrangement of 

 sedimentary formations is parallel. It is to be expected, therefore, that 

 we should find the demonstration of volcanic force along a line nearly 

 parallel to that of maximum sedimentation. This is in reality the case. 

 Where the process of sedimentary deposition is still going on, we ob- 



* Dall, Alaska and its Resources, 1870. 



