516 Grout — Lopolith; An Igneous Form 



Art. XX. — The Lopolith; an Igneous Form Exemplified 

 by the Duluth Gabbro; by Fkakk F. Grout. 



Contents. 

 Introduction. 



Possible forms of the Duluth gabbro, and early suggestions. 

 The laccolith. 

 The lopolith. 



General remarks on the Duluth gabbro. 

 Summary. 



Introduction. — The several students of the Duluth gab- 

 bro as a formation have had several opinions as to its 

 form and relations. Eecent descriptions refer to it as a 

 laccolith, though it differs from the typical laccoliths in 

 some details. Several other large intrusions are of sim- 

 ilar form, and it is here suggested that the form deserves 

 a special name. The size and relations of the Duluth 

 mass are summarized. 



Possible forms of the Duluth gabbro, and early sugges- 

 tions. 1 — In 1883, R. D. Irving referred to the gabbro as 

 probably the reservoir from which the Keweenawan flows 

 came. N. H. Winchell, in several papers from 1880 to 

 1910, refers to the "great basal flow" and later to bosses 

 and intrusive masses. Bayley, as late as 1893, quotes 

 Irving that it is "not intrusive in the ordinary sense," 

 but says it might be a succession of thick flows or the 

 reservoir from which the flows came. Grant, in 1900, and 

 others more recently have described it as a laccolith. 



The intrusive character of the gabbro is clearly shown 

 at Duluth. 2 It has as definite a roof and floor as a lac- 

 colith or sill, and was intruded along a surface approxi- 

 mately corresponding to a previous structure, — the 

 unconformity at the base of the Keweenawan. On the 

 basis of its banded structure one may estimate the posi- 

 tion of its floor. This eliminates the probability of any- 

 thing funnel like or particularly irregular, — it is not like 

 the "ethmolith" or "chonolith." Thus it comes about 

 that by a process of elimination the gabbro is placed with 



1 Irving, R. D., Copper bearing rocks of Lake Superior : TJ. S. Geol. Surv., 

 Mon, 5, pp. 144, etc. 



Winchell, N. H., Minn. Geol. and Nat. Hist. Survey, Ann. Kept. 10, p. 

 114, 1881 ; Final Eept., vol. 4, and vol. 5. 



Grant, U. S., Minn. Geol. and Nat. Hist. Survey Final Kept., vol. 4, p. 326 ; 

 and Bull. Geol. Soc. America, vol. 11, p. 505, 1900. 



2 Grout, F. F., Paper at the December (1917) meeting of the Geological 

 Society of America. 



