6 J. Barrell — Relations of Subjacent Igneous 



associated with regional metamorpliism and themselves 

 show various stages of mashing. Some, such as those of 

 eastern Massachusetts, are intruded into Carboniferous 

 schists and must have reached comparatively near to the 

 surface; others, especially many of the pre-Cambrian 

 granites, must have solidified at profound depths, since 

 the portions now exposed have been uncovered through 

 many cycles of uplift and erosion. 



It is especially with this group, which is illustrated in 

 the regionally metamorphosed province of New England, 

 and which may be called or o genie batholiths, that this 

 article is planned to deal. 



Underground Extension of Cordilleran Batholiths. 



To estimate the part which batholithic invasion may 

 play in regional metamorpliism, it is necessary to form 

 a conception as to the amount of broadening which they 

 may take on with depth. The first step in observation 

 and inference is from the nature of the margins. These 

 in places show broad, flat or domal, or irregular roofs ; 

 in other places, contacts plunging steeply for thousands 

 of feet. The general character, however, is a broadening 

 with depth. Inference from the nature of the outcrops 

 can not safely lead us very far, but it does show no evi- 

 dence of a bottom, no narrowing into pipes or laccolithic 

 form. The doming of the roof would result from the 

 hydrostatics of the magma, without any need of a floor. 

 Being lighter than the solid rock around it, the batholith 

 must perforce exert an upward pressure, and a pressure 

 greater in proportion to the thinness of the cover. 



The next step downward is taken by noting the relation 

 of cupolas to the main body. The roof is seen to be 

 irregular, and beyond the margin satellitic stocks break 

 up and are intersected by the erosion surface. Erosion 

 to a greater depth would join these to the parent body, 

 and show at farther distances a new set of satellitic 

 stocks. The location of these latter can be recognized in 

 places by the centers of outlying metamorphism or 

 centers of converging dikes. 



On a larger scale, small batholiths are seen to occur 

 more or less irregularly in regions adjacent to the greater 

 ones. A good illustration is seen in the great batholith 

 of central Idaho and its relation to those of southwestern 

 Montana. They transgress rocks of various Mesozoic 



