554 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



Soda-rhyolites of Tertiary age have been described from Cali- 

 fornia by C. Palache.* 



Trachytes occur in the Elk Mountains and Wasatch Mountains 

 of the South-western States, but they have never been shown, to 

 my knowledge, to belong to the alkaline division. 



Wherever rocks of true foyaitic composition have been described 

 in North America, they occur on the faulted outskirts of that 

 continental massive which has preserved a passive character, 

 from the Cambrian to the present day, over which the sediments 

 deposited by Silurian, Carboniferous and Cretaceous transgres- 

 sions remain almost horizontal. The regions in which they occur 

 have not, since the Cambrian and Lower Silurian, been subjected 

 to any considerable sedimentation, and certainly to no tangential 

 movements. 



Alkaline rocks appear rare or wanting in regions subjected to 

 true folding-movements in late Palaeozoic, Mesozoic, or Tertiary 

 times. 



In addition to the areas hitherto mentioned there are very 

 many other alkaline areas in the United States which without 

 exception occur in such surroundings as have been described as 

 typical for alkaline rocks. 



The following are the most important of them : — 



(1) The Petrographical Province of Essex County, Mass., 

 described by Dr. H. S. Washington.! 



(2) Red Hill and the Belknap Mountains, New Hampshire, 

 described by L. V. Pirsson and H. S. Washington. J 



(3) The Little Rocky Mountains in Central Montana, described 

 by W. H. Weed and L. V. Pirsson.§ 



(4) The Bearpaw Mountains, Montana, reviewed by W. H. 

 Weed and L. V. Pirsson. || 



* Bull. Dept. Geol. Univ. Calif. Vol.i. p. 61. 

 + Journal of Geology, Vol.vi. No. 8, and Vol.vii. Nos.l, 2, 3 and 5; also 

 Amer. Journ. of Sci. Vol.vi. 1898. 



X Amer. Journ. of Sci. Vols.xx., xxii. and xxiii (1905, 1906, 1907). 



§ Journ. of Geol. Vol.iv. No. 4; also Amer. Journ. of Sci. Vol.1. 



II Amer. Journ. of Sci. Vol.ii. 1896, and Vol.i. 



