JP-irsson and Washington — Geology of JVeiv Hampshire. 351 



which broke up at the border of the diorite, involving masses 

 of it in its various modifications and thus produced the breccia. 

 If this view is adopted, there are but three periods of eruption 

 and they follow the normal course commonly seen in such 

 cases. 



Dikes. — It has been observed by us that wherever the main 

 types of igneous rocks are exposed over considerable areas in 

 this mountain tract they are commonly cut by dikes, and the 

 same is true of the border zone of the enclosing schists and 

 gneisses. Except, however, in the highest parts of the moun- 

 tains, such exposures are not very common nor are they of 

 great size. It seems probable, therefore, that only a very small 

 part of the dikes actually present in the region has been seen 

 by us, the greater part being covered up by the heavy mantle of 

 debris and glacial drift. 



As is so often the case when the dikes are found to be a 

 throng of satellites attendant upon a large body of igneous 

 rock, they may be readily referred to two strongly contrasted 

 groups. In one of these the rocks are light colored, strongly 

 persalic and therefore almost devoid of ferromagnesian min- 

 erals ; in the second the rocks are dark colored, salfemic, 

 heavy and composed in very large, if not for the greater part, 

 of ferromagnesian minerals. They are persalanes and salfe- 

 manes in the new classification or aplites and camptonites in the 

 older ones. 



The persalane dikes are found cutting the main syenite in 

 all directions, of a generally pink color and varying in size 

 from dikelets but a few inches in breadth to masses twenty feet 

 wide. The bare exposed slopes and ledges of the upper part 

 of Mt. Belknap were found cut by them in great abundance 

 and it was here noticed that they often ended abruptly and 

 appeared as if somewhat elongated roughly lenticular masses. 

 They were often branched, were connected with others, anasta- 

 mosecl or formed reticulated systems, large and small together. 

 Their small angular chippy jointing, light color on the weath- 

 ered surface and flinty felsitic aspect clearly distinguished 

 them from the massive granular rock they cut. These same 

 characters were found repeated on the exposed surfaces of 

 Mt. Gunstock and Mt. Piper, and in one place, about half 

 way up Mt. Gunstock from Morrill's house, on the west slope 

 above the spring, the ledges in a pasture field on an open 

 shoulder of the mountain were found cut by a dike of this 

 nature 15-20 feet in width and with north and south trend. 

 It was also found that where the contact zone was exposed at 

 the foot of the mountain slopes, as along the west side in the 

 localities described above, that both the igneous rock and the 

 enclosing schists and gneisses were cut by dikes and stringers 



