Portsmouth Basin, Me. and N. H. 155 



the composition of a quartz diorite or an alkaline granite 

 if the magma in question were molten at a temperature 

 even less than 1000° C. The stoping hypothesis 17 may 

 thus account for a large measure of the final emplacement, 



Although assimilation in place may have removed a 

 part of the invaded sediments, and although block-fault- 

 ing or differential lateral movement may account for a 

 part of the width of these batholiths, nevertheless, the 

 areas of contact breccias, the roof pendants, and the 

 relative specific gravities of molten rock and engulfed 

 fragments all seem to indicate that the intrusives of the 

 Portsmouth Basin in their later stages have been 

 emplaced to a large degree by magmatic stoping. 



Slightly different in its method of emplacement is the 

 Cape Xeddick gabbro. Excellent exposures of this body 

 are to be had, the northwestern contact being particu- 

 larly illuminating. Here the waves have cut a marine 

 bench about 100 feet wide and at low tide the rocks can be 

 studied in detail. Basic and granitic dikes cut the Kit- 

 tery quartzite which forms the country rock. Close to 

 the intrusive the dikes and sediments are mixed by 

 contact brecciation and contact metamorphism has almost 

 obliterated the original lithologic differences. A short 

 distance from the contact the sediments are seen to be 

 crumpled, simple folds, overturned folds and overturns t 

 faults all being present on a miniature scale. The crump- 

 ling gives way 75 feet distant from the contact to gentle 

 folding and this in turn grades into the characteristic 

 steep dip and NE-SW strike of the sediments. The intru- 

 sive was apparently emplaced in part by bodily forcing 

 its containing walls apart and crumpling and mashing 

 them. As shown by the peculiar quartz-rich contact 

 phases a part of this shattered rock was assimilated; 

 a part seems also to have been removed by being floated 

 away. This latter condition is well illustrated by frozen- 

 in isolated blocks of quartzite weighing several tons now 

 removed a short distance from the contact. The figures 

 dealing with specific gravities indicate that these blocks 

 may have sunk in gabbroid magma, there to be slowly 

 digested and assimilated at depth. 



17 For a detailed presentation of this hypothesis, see E. A. Daly, Igneous 

 Eocks and Their Origin, p. 194, 1914. 



