698 REGINALD ALDWORTH DALY 
From this brief review it is seen that the placing of the 
‘“porphyritic gneiss’ so low in the geological scale was largely 
due to the prevalence of two pernicious doctrines then held in 
the study of crystalline schists. The application of the ‘‘litho- 
logical canon”’ was a constant feature in the efforts of the sec 
ond survey to work out their difficult field. The coarse granitic 
eneisses, the augen-gneiss, and the andalusite-gneiss were each 
supposed to be represented in the typical Laurentian of the 
better known regions, thereby establishing synchrony. Again 
the distinct foliation in many parts of the ‘‘ porphyritic gneiss ”’ 
led to the other serious error of considering the rock as a meta- 
morphosed sediment, which still preserved traces of its original 
planes of stratification.* This position being taken, it was but 
natural to look for structural relations with the surrounding for- 
mations, and at many contacts, the greater antiquity of the por- 
phyritic rock would often appear evident. Needless to say, how- 
ever, in the light of present knowledge, that all such reasoning 
is without foundation so far as it refers to large isolated areas of 
thoroughly crystalline schists. Thus the character of the 
terrane had to be determined by other methods. The latter were 
very sparingly used by the survey, and consequently its final con- 
clusions assigned to the ‘‘porphyritic gneiss” the very important 
position of a foundation member in the entire geological series. 
The interpretation of other terranes was, of course, greatly influ- 
enced by this fundamental idea. The survey fixed the geologi- 
cal position of the Bethlehem gneiss,? and of the Montalban 
group and the Lake Winnipiseogee gneiss 3 directly by reference 
to the ‘‘porphyritic gneiss,” and the later succession was corre- 
spondingly affected. In fact, Professor T. Sterry Hunt’s con- 
ception of and nomenclature of the Montalban group was 
founded on the conclusion that there is this demonstrable Lau- 
rentian in New Hampshire. 
' Geol. of N. H., Vol. II, p. 99. 
Zitis\7ily sae Ceol, Ot ING Jala Wolk Il, jos B43 Wolk Wh joy AS. 
3 [bid., Vol. II, pp. 564,662. 
4 Geol. Mag., 1887, p. 1499; Nature, Sept., 1888, p. 52. 
