Exemplified by the Duluth Gabbro. 517 



the laccoliths. It is best, however, to review the defi- 

 nitions and usage of the term laccolith. 

 ■ The laccolith. — The laccolith as originally defined by 

 Gilbert 3 is insinuated between strata (or along the plane 

 of some previous structure) with a flat floor and an 

 up-domed roof ; its thickness ranges around one-seventh 

 its width, and its ground plan is nearly circular. Several 

 geologists, after wide experience with intrusive masses 

 elsewhere, have found it convenient to slightly modify the 

 definition to include clearly related masses. 4 Thus, the 

 concordance with previous structure is not always per- 

 fect, but a general tendency is characteristic; the form 

 also may be somewhat unsymmetrical. Laccoliths grade 

 into sheets on one hand, and into "bysmaliths" with 

 faulted uplifted roof, on the other. It seems to have 

 been agreed that the magma was aggressive in uplifting- 

 its roof, stretching the overlying beds and separating its 

 roof and floor; Harker even coined the name "Phaco- 

 lith" for similar forms which might be attributed to other 

 forces. 5 



Several large intrusions are known which differ from 

 laccoliths in having a sunken rather than a domed roof ; 

 in fact, some are so thick that a roof could not have been 

 held up, isostatically. The masses are now in the form 

 of great saucers or basins. The process of intrusion was 

 probably very different from that of a laccolith. In spite 

 of the fact that each of the several examples has in recent 

 years been described as a laccolith, it is difficult to formu- 

 late a definition to include both types. For example, 

 Daly gives an excellent summary of current usage, and 

 defines a laccolith as plano-convex or doubly convex. 6 

 Later he calls the larger concavo-convex masses lacco- 

 liths, frankly admitting that they are departures from the 

 type. 



This being the case, Professor Joseph Barrell has sug- 

 gested that as igneous forms they deserve a distinct 



3 Gilbert, G. K., Eeport on the geology of the Henry Mountains, IT. S. 

 Geol. and-Geog. Survey of the Eocky Mountain region, pp. 19, 53 and 55. 



4 Geikie, A., Structural and field Geology, p. 190. 

 Iddings, J. P., Igneous rocks, vol. 1, p. 314. 



Harker, Alfred, Natural history of Igneous Eocks, p. 65. 



Pirsson, L. V., and Schuchert, Charles, Text book of Geology, pt. 1, 

 p. 297. 



6 C. E. Keyes, however, now argues for a different mechanism for the 

 true laccolith, December (1917) meeting of the Geol. Soc. of America. 



6 Daly, E. A., Igneous Eocks and their Origin, p. 70. 



