350 Hlrsson and Washington — Geology of New Hampshire. 



it to be a later intrusion than the syenite and that it has broken 

 up alongside of it. The upper contact with the syenite is, 

 however, largely replaced by a remarkable breccia zone to be 

 presently described. This rock varies considerably in charac- 

 ters from place to place, as will be described in a succeeding 

 petrographic paper. 



Breccia zone. — As just mentioned, the contact between the 

 diorite and syenite above is occupied by a brecciated rock mass. 

 The cement is a quartz-alkali feldspar rock much like the 

 granite facies previously described ; it has a sugar granular 

 texture, and is of the character of rocks designated as aplites. 

 In this are thickly scattered blocks of all sizes, which may 

 attain an extreme dimension of four feet in length but which 

 average perhaps a foot or two in diameter and descend from 

 this size to minute fragments of a fraction of an inch. In 

 some places they are so thickly crowded that their mass is 

 much greater than that of the cement. In shape they are 

 usually extremely angular and the sharpness of the angles has 

 been perfectly preserved. In other cases they appear some- 

 what rounded as if partially melted, and are surrounded by 

 darker aureoles richer in ferromagnesian minerals. It is 

 remarkable, on the other hand, how some of the smallest frag- 

 ments retain in some cases all their distinctness of outline. 

 There are several different types of rocks among these included 

 fragments. One common one is a dense black basaltic-looking 

 type too compact for the component minerals to be seen, in 

 which lie phenocrysts of mica and other minerals — a rock of 

 well defined lamprophyric character. Other fragments are of 

 the diorite mentioned above, while others are obviously pieces 

 of gneisses and schists. 



The determination of the relative age of this breccia and of 

 the syenite and diorite is not easy. It would be simple to 

 imagine that the latter is the older rock, that the syenite with 

 its granite border broke up alongside of it enclosing masses of 

 femic rock. If this view is adopted, then the basaltic, lampro- 

 phyric and granitic and felsitic dikes which cut the syenite 

 must of course be separate and later intrusions and there would 

 be four periods of eruption, in two of which, those of the 

 diorite and the lamprophyric dikes, similar magmas were pro- 

 duced. The oldest rock, the diorite, is then also the most dif- 

 ferentiated one, a fact contrary to general experience. Con- 

 sidering these points, we are inclined to believe the syenite the 

 first and oldest, to place the eruption of the diorite next, which 

 would also explain the distinct endomorphic contact modifica- 

 tion it exhibits toward the syenite and make it contemporane- 

 ous with the lamprophyric dikes. Then came an eruption of 

 granitic magmas, which also forms dikes in the syenite, one of 



