210 SCIENCE PROGRESS. 



To the sill and the laccolite Russell (15) has added 

 another type of intrusion, to which he gives the name 

 plutonic plug, constituting the third term of a genetically 

 related series. As the laccolite differs from the sill in the 

 narrower localisation of the intrusion and of the resulting 

 uplift of the overlying strata, so in the plutonic plug this 

 localisation is carried to an extreme degree, and the form of 

 the igneous rock-body becomes cylindrical rather than dome- 

 shaped. The intrusions taken as typical examples occur in 

 Wyoming and South Dakota, where they have given rise to 

 a number of hills, some having very striking outlines. The 

 hill named Mato Tepee is of columnar shape, rising almost 

 vertically to a height of more than 600 feet above the plat- 

 form at its base, and composed of igneous rock thrust 

 like a plug through the strata of the district. The latter 

 are horizontal and undisturbed, even up to the very base of 

 the tower. Other examples are more or less completely 

 exposed, and in some localities, as at Little Sun Dance 

 Hill, there is only a dome-like elevation of the strata to 

 indicate a plug entirely concealed. Comparison of the 

 different cases makes it clear that the igneous magma never 

 reached the surface, and in other respects the intrusions 

 differ entirely from volcanic "necks". The texture of the 

 igneous rocks gives evidence of consolidation at a consider- 

 able depth, and the geological relations of the district war- 

 rant the same conclusion. The arching of the strata over 

 the concealed plug of Little Sun Dance has been effected 

 without fracture, which argues a very great superincumbent 

 load, and the absence of dykes throughout the region is 

 significant in the same sense. As regards the petro- 

 graphical character of the intrusions, there is some obscurity 

 in Russell's account, but the rock of Mato Tepee has 

 recently been examined by Pirsson. It is of phonolitic 

 composition, with 6ro8 per cent, of silica and 1871 of 

 alumina. We may safely infer that the magma was of a 

 highly viscous nature, probably comparable with those 

 which in volcanic outpourings formed the trachytic lava 

 domes of Auvergne and the phonolitic protrusions of that 

 and other districts, to which may be added the curious puy- 



