SO-CALLED PORPHYRITIC GNEISS OF NEW HAMPSHIRE 699 
We have already referred to some hints of an igneous, 
intrusive origin for the ‘‘ porphyritic gneiss,” that were given by 
the survey officers. Similar suggestions appear in many parts of 
the different reports.t The facts described in these passages 
were supposed to be explained on the metamorphic theory as 
being characteristic of only those parts of the ancient stratified 
rocks which had been altered to the extent of complete fusion. 
In his address as vice president of section & of the Ameti- 
can Association in 1883, Professor Hitchcock expressed some 
modification of his earlier opinions on the origin of the por- 
phyritic gneiss. Hesaid: ‘A careful study of the crystalline 
rocks of the Atlantic slope indicates the presence of scattered, 
ovoidal areas of Laurentian gneisses. Those best known have been 
described in the geology of New Hampshire. Instead of a few 
synclinal troughs filled to great depths with sediments, the oldest 
group is disposed in no less than twenty-two areas of small size, 
scattered like the islands in an archipelago. There are no 
minerals in these Laurentian islands that do not occur in eruptive 
granite ; and the schistose structure is often so faint that the 
field geologist need not be blamed if he acknowledges his inability 
to detect it. Likewise we discover the same fluidal inclusions 
and the vacuoies that pertain to granite.’3 Comparing these 
islands to volcanic oceanic islands of the present day, he suggests 
that the foliation of the porphyritic gneiss may be the result 
of the superposition in quaquaversal sheets of lava about each vol- 
canic cone, aided by flows of mud and wear by water between 
igneous flows. In this way we might have a ‘“ concentric stati- 
form arrangement in the whole mass.’’ Subsequent metamor- 
phism by heat and pressure would lead to the development of 
new minerals in foliated beds. 
The next important notice of this formation appears in Whit- 
ney and Wadsworth’s ‘‘ Azoic system.’’* They instituted a close 
“isis Ceol, GEING Isl, Il Dh B75 BSB IWS eh Ow, aya, Gack 
2RLOC WAAL EAG SEL OOR WE DaloO. 
S1Op cite, Palo. 
4 Bull Mus. Comp. Zool., Harvard College, Vol. VII, 1884, p. 383 ff. 
