266 J. BarreJl — Relations of Subjacent Igneous 



of this igneous activity would seem to be late Carbon- 

 iferous, as the sediments of the latter age in Rhode Island 

 and Massachusetts have been intruded and metamor- 

 phosed. Warren and Powers-^- hold, however, that some 

 of the eastern Massachusetts granites belong to the 

 Devonian orogenic movement. 



The relation between the different intrusives and the 

 stages in regional metamorphism in eastern Connecticut 

 has been well presented by Loughlin.^^ Here the intru- 

 sion of a gabbro laccolith preceded the regional metamor- 

 phism, since the blocks of wall rock included within the 

 resistant gabbro show as angular blocky fragments of 

 hornfels, whereas the adjoining country rock is now 

 highly foliated. The granite of the same area appears 

 to have been intruded in association with the regional 

 mashing and to have completely crystallized before it 

 ceased. The alaskite, the youngest rock, completed its 

 crystallization after the closing of the period of rock 

 mashing. 



The conclusion of first imiDortance which a statement of 

 these relations leads up to is that the regional metamor- 

 phism is intimately related to the batholithic intrusion. 

 Deformation and metamorphism of the sediments began 

 before the intrusion of the granites, but had not begun 

 before the intrusion of the Preston gabbro. It ceased be- 

 fore the last upbreaks of the acidic phases of the magma. 

 This conclusion applies to all southern New England 

 and apparently much the same relationship between intru- 

 sion and metamorphism existed in the Laurentian 

 invasion. 



Furthermore, both igneous activity and crust move- 

 ments are known to be of a periodic character. The 

 suggestions of this study are that in regional metamor- 

 phism there is a certain intimate rhythm between the two. 

 The phenomena of injection and mashing are alternating 

 phases in a general period of intrusion and metamor- 

 phism. When the pressures in the upper part of the 

 magma come to exceed the lateral pressure on the cover 

 rocks, the advance gases, insinuating themselves on 

 foliation planes, widen them out and permit the ascent of 

 the regional magma. This favors mashing of the 

 adjacent region. But when the yielding of adjacent 

 parts of the crust concentrates the lateral compressive 



®^ Personal communication. 



3^ G. F. Loughlin, U. S. Geol. Survey, Bull. 492, 1912. 



