, t,y^ "—^ r.- ■ —. — •. __ _ r m-^-rr^r^. n 



424 



THE AZOIO SYSTEM AND ITS SUBDIVISTOXS. 



the entire formation seems to have been fluent, and the extravasation has been 

 so extensive that the character of the rock changes nearly every rod. One im- 

 portant fact should be noted here, viz. : nowhere in this region does the 

 Norian series appear to be cut by eruptives belonging to another hjrmutionj for 

 all the extravasated rocks of this system may be easily referred to, or shown to 

 be derived from, its stratified members At Nahant we find slates, be- 

 lieved to be of Primordial age, resting upon the Norian diorites, wliich have 

 been extravasated through the slates, producing extensive alterations. The 

 coarse grained, readily disintegrating, exotic diorites, so extensively q uariied in 

 Medford, and also occurring in Somervillc and Brookline, are, doubtless, ex- 

 truded portions of this same series, which is the probable scat of many of the 



eriiptive rocks, especially diorites, cutting the newer formations Here 



is the real base of our geological column." (Report, 1876, pp. 8, 10, 11.) 



Wo have in Mr. Crosby's work above quoted strongly formulated the 

 principle that any dike cutting another rock is older than the rock it 

 cuts, and is itself of distinctly recognizable age, — a perfectly logical 

 result of Dr. Hunt's teachings. The "Norian" dikes in Mcdford, 

 Somerville, and Brookline were found by Dr. Wadsworth to be diabase 

 (Proc. Bost. Soc. Nat. Hist., 1877, XIX., pp. 217-237), and to belong to 

 old basaltic eruptions. We must then conclude, according to this view, 

 that all basaltic rocks, even the outflows of modern volcanoes, arc de- 

 rived from the Norian system, and are to bo mapped and called Noinan. 

 To be sure we find such dikes cutting the Laurcntian of other regions, 

 but in this latter case they were doubtless extravasated downwards, 

 as advocated recently by Professor Shaler (Mora. Vol. Bost. Soc. Nat. 

 Hist., pp. 3-15). Dr. Wadsworth has also carefully studied the dikes 

 both in the so-called Laurcntian and Huronian, and can aillrm their 

 unity, both in macroscopical and microscopical characters. 



Mr. Crosby divided his supposed Huronian rocks into the following 

 scries : hornblcndic granite, fclsite, diorite, stratified rocks, and lime- 

 stone. Of the first series it was said : 



" That these granites are mainly exotic, can scarcely be questioned, for we 

 have seldom far to look, to find, in the form of enclosed angular fragments of 

 clearly stratified rocks, evidence of their extravasation, and near tlie bounda- 

 ries of the granites we usually find them cutting the adjoining rock, especially 

 if that is stratified, in a manner incompatil)le with any theory that wouhl re- 

 gard them, in their present condition, as indigenous or endogenous. Never- 

 theless it is doubtless true, as suggested by Proi'. N. S. Shaler, and later by 

 Mr. T. T. Bouve, that tliese granites have been derived from sedinicuUiry 

 rocks, and have simply reached the final term in the metamory)hic process -— 

 igneous, or more probably, igneo-aqueous fusion." (Report, 1876, p. 14.) 



