THE NATURAL HISTORY OF IGNEOUS ROCKS. 209 



the rocks, and suggests that basis laccolites may have been 

 overlooked owing to their not giving rise to salient topo- 

 graphical features ; but the facts, so far as they are known, 

 seem to point to a more positive conclusion. Indeed Gil- 

 bert in his original memoir on the Henry Mountains laid 

 stress on "the coincidence of the laccolitic structure with a 

 certain type of igneous rock," and based on this his theory, 

 which can scarcely be maintained at the present day, that 

 the behaviour of the intrusions depended upon " the relative 

 densities of the intruding lavas and of the invaded strata'". 

 As regards the question of load, it seems to be generally 

 agreed that typical laccolites were in all cases formed under 

 a very heavy cover of overlying strata. The stratigraphical 

 evidence pointing to this conclusion is reinforced by the 

 results of Cross's petrographical study of a large suite of 

 specimens. Another point noticed by Gilbert in the Henry 

 Mountains is an apparent relation between laccolites and 

 the nature of the encasing strata : they are always enclosed 

 by soft beds. This is, however, to be considered not so 

 much a condition of the formation of laccolites as a factor 

 in determining their selection of a horizon. It does not 

 apply to the associated sheets and dykes. 



Excluding the modifications observable in laccolitic 

 intrusions among strata either folded or in process of fold- 

 ing, there are still occasional departures from the simple 

 form of the regular dome with flat base. One such special 

 development is seen when the intrusive mass has spread, 

 not along one, but along many bedding-planes, the central 

 mass passing laterally into a number of rapidly thinning 

 sheets. Instead of the ideal mushroom form, there results 

 one which is comparable rather with a cedar-tree. This, 

 according to Holmes, is exemplified in the La Plata 

 Mountains of Colorado, and Geikie has drawn a similar 

 section to represent the relations of gabbro masses injected 

 among the bedded basalts of Skye. In the latter case a 

 connection seems to be suggested between the partial re- 

 version to the sill form and the basic nature of the intruded 

 magma, but the relatively acid composition of the La Plata 

 rocks forbids our laying much stress on this point. 



