700 REGINALD ALDWORTH DALY 
criticism of the general methods of the New Hampshire survey, 
and looked with especial disfavor upon the liberal use of the 
as accepted by the members of that survey. 
” 
“lithological canon, 
A convenient résumé of the various classifications proposed for 
New Hampshire rocks is given in tabular form at page 396 of 
the memoir. The possibility of an eruptive origin for the 
‘“‘porphyritic gneiss ’’’ was remarked by the authors. 
In 1884 Professor Hitchcock stated that ‘all thoroughly 
crystalline series of the Atlantic region are of Eozoic age.’’* 
Two years later he edited the ‘‘ Geological Map of the United 
States,” in which the “‘porphyritic gneiss” is colored as Lauren- 
tian.2 A somewhat full account of his opinions on the forma- 
tion was given ina paper on the “ Significance of Oval Gran- 
itoid Areas in the Lower Laurentian,’3 from which it is evident 
that in 1890 Professor Hitchcock held practically the same views 
on the present subject as those which he published in 1883. 
While the present paper was in process of preparation the 
last word on the formation was given by Professor Hitchcock in 
this JourNnaL. After tracing in a general way the history of 
geological surveying in New Hampshire, he makes the following 
significant statement of a changed point of view: ‘“‘The question 
now arises, how can our early classification [ of the rock series | 
be improved? It is eighteen years since the New Hampshire 
report was published, and there are many new workers in the 
field, all placing great reliance upon petrographical principles, 
such as were inaugurated in Dr. Hawes’ report. Some are 
advocates of extreme metamorphism, and hence the conclusions 
are not harmonious. It seems to us that our early views may 
be modified by the following principles: (1) The mineral 
characters of crystalline rocks are not a sure guide to geological 
age. (2) Protogenes, diabases, and diorites, more or less inter- 
stratified with hydro-micas, are of true igneous origin. (3) The 
*Trans. Am. Inst. Min. Eng. XII, 1884, p. 68. Cf. a paper in Proc. A. A.A.S. of 
the same year (p. 396) where he again holds the porphyritic gneiss to be Laurentian. 
? Trans. Am. Inst. Min. Eng., Vol. XV, 1886, p. 465. 
3 Bull. Geol. Soc. Am., I, 1890, p. 557. 
