BYSMALITHS. 17 



core or plug. It is a type of intrusion for which the name bysmalith has 

 been proposed. 1 



A laccolith as defined by Gilbert 2 is a body of igneous rock which has 

 insinuated itself between two strata and opened for itself a chamber by 

 lifting all the superior beds. A symmetrical dome-shaped body is the 

 exceptional or ideal form, and, as Cross 3 has pointed out, Gilbert's use of the 

 term practically included all thick lenticular masses of intrusive igneous 

 rock occurring at a certain geological horizon in a sedimentary complex. 

 Cross includes under the term laccolith all masses in which the expansion of 

 the body has taken place from a plane approximately parallel to the bedding,, 

 and says that numerous causes may affect the regularity of the form. 

 Of these the principal are: (1) Oblique position of the plane of expansion 

 to bedding planes of the sediments; (2) lines of structural weakness in the 

 strata; (3) presence of earlier intrusions; (4) lack of coherence and of 

 pronounced bedding in strata invaded. These factors, we understand, simply 

 modify the form of the laccolith, whose essential characters are those 

 described by Gilbert. They can not in any sense replace the latter. 



A laccolith is distinguished from an intrusive sheet of igneous rock, 

 which is an intrusion between strata accompanied by a certain amount of 

 lifting of the superincumbent rock. The difference lies in the thickening 

 of the igneous body into a more or less lenticular mass in the case of a 

 laccolith, over which the strata arch ; whereas the upper and lower surfaces, 

 of a sheet are almost parallel to each other. In sheets the lateral dimen- 

 sions are very great as compared with the depth or thickness; in laccoliths 

 the difference between the thickness and the lateral dimensious is much less. 



Cross has shown that a certain amount of vertical displacement may 

 accompany the arching of the overlying strata, as in the laccolith of Mount 

 Marcellina, 4 without changing the general character of the intrusion. But 

 where vertical displacement with faulting is one of the chief characteristics 

 of the intrusion, a distinction from normal laccolithic intrusion should be 

 recognized. In the extreme this would result in the forcing upward of a 



i Iddings, J. P., Bysmaliths: Jour. Geol., Vol. VI, 1898. 



2 Gilbert, G. K., Report ou the Geology of the Henry Mountains, U. S. Geog. and Geol. Surv. 

 Rooky Mountain region (J. W. Powell in charge), 1877, p. 160, PL V. 



'Cross, Whitman, The laccolithic mountain groups of Colorado, Utah, and Arizona: Fourteenth 

 Ann. Rept. U. S. Geol. Survey (for 1892-93), 1895, p. 236. 



•' Loc. cit., p. 236. 

 MON XXXII, PT II 2 



