208 . SCIENCE PROGRESS. 



The typical laccolite, using the term in its original and strict 

 sense, is associated with the plateau, not the mountain, kind 

 of structure, and the most perfect examples of it have been 

 described from the great plateau region to the west of the 

 Rocky Mountains. Its special feature, as contrasted with 

 the other types of intrusion to which the name laccolite has 

 been extended, is that the strata beneath are undisturbed, 

 while those above are elevated into a dome, this elevation 

 being due directly to the intrusion itself. The simple lacco- 

 lite thus differs from the uniform sill in its more restricted 

 area and greater relative thickness at its centre, the len- 

 ticular mass thinning out rapidly in every direction. In 

 other words, the elevation of the overlying strata, instead 

 of being extended and of small vertical amount, is localised 

 and correspondingly accentuated. The facts elicited above 

 lead us to inquire whether this difference is connected with 

 a higher degree of viscosity in the magma and a greater 

 thickness of superincumbent strata. As regards the former 

 condition at least the answer seems to be in the affirmative, 

 the typical laccolites being of acid and intermediate rocks. 

 Those of Colorada, Utah and Arizona, as described (6) by 

 Whitman Cross, have silica-percentages ranging from 56*62 

 to 73'50, with an average of 63^84 : the percentage of potash 

 varies between 1*97 and 3*95, the average being 3*02. 

 Rock-magmas of such composition would be much more 

 viscous than the diabases and basalts which are so familiar 

 in the form of sills. It is true that Gilbert (16) has recorded 

 laccolites at Twin Butte, in the south-eastern part of Colo- 

 rado, composed of what he characterises as a basic syenite- 

 porphyry, but no account of this rock has yet been pub- 

 lished, 1 and the limited exposures make the form of the 

 intrusions a matter of inference rather than demonstration. 

 The author named rejects any generalisation as to the 

 connection of the laccolitic form with the composition of 



1 If this is the rock from " Two Buttes," of which an analysis has since 

 been given {Bull. No. 148, U.S. Geo/. Sur., p. 182, 1897), it seems to be 

 allied to tinguaite. Though poor in silica, it contains 4/08 per cent, of 

 potash and 6-70 of soda, and its magma would doubtless possess a high 

 degree of viscosity. 



