EASTERN MASSACHUSETTS. 



421 



ately to a dilTerent name, this is certain, that some of them are found cutting 

 tlie slate and conglomerate, and poured out over them, and that near the con- 

 tact the slate ia greatly hardened." 



Of the '* stratified rocks " Mr. Dodge remarks : 



*' Between the older rock bands described lie certain more recent and more 

 clearly stratified rocks. The more recent age of these is shown beyond dis- 

 pute hy their position in relation to the underlying crystalUnes, as well as by 

 the fact that they are composd of detritus of the latter, which may not only be 

 so recognized, but even may at times be referred to the source from which it 

 was probably derived." 



The "stratified rocks" were separated into slates and conglomerates, 

 although much argillite was united with the latter. The "slates" in- 

 cluded not only the Braintree ai-gillite, but also many of the other argil- 

 lite deposits, and in common were referred to the St. John group. He 

 further states : — 



" In all the localities they are much disturbed by intrusions of igneous rock 

 (which is sometimes poured out over them), are faulted, hardened and dis- 

 torted." 



Mr. Dodge considers the conglomerate as overlying the Braintree 

 argillite, and as being probably . of Carboniferous age. (Proc. Best. 

 Soc. Nat. Hist., 1875, XVIL, pp. 411, 412.) Dr. Hunt states in the 

 same volume that he regards the argillite about Boston Harbor as 

 being of the same ago as the Braintree argillite, (Ibid., pp. 48G--488.) 



The following is a quotation embodying rcmarlcs made by Dr. Hunt, 

 at a meeting of the Boston Natural History Society, in the same year : 



*' The crystalline rock, .... seen in contact with the fossiliferous Lower 

 Cambrian (Menevian) strata of Braintree, Mass., is clearly a variety of tlie feld- 

 spar-porphyry or orthopyre, which is so abundant along the eastern coast of 

 MsvSachusetts, Maine and New Brunswick, and which passes on the one hand 

 into a jaspery petrosilex, and on the other, into a finely granular, almost grani- 

 toid rock The porphyries of Lynn, Marblchcad and Salem, and the so- 

 called jaspers of Saugus and Newbury, belong to it. This rock is identical with 

 the porphyry which accompanies the crystalline iron ores of southeastern Mis- 

 souri, and is also well displayed on the north shore of Lake Superior. It is, 

 in all these localities, distinctly stratified, and has been by the speaker referred 



to the Huronian series of rocks This porpliyry, in the form of pebbles, 



often forms conglomerate beds in the Keweenaw or copper- bearing series of 

 Lake Superior, as is well seen in the Calumet and Ilccla, and the Boston and 

 Albany mines. 



" As regards the relations of the eruptive granites of our Eastern coast to the 

 Braintree fossiliferous slates, Dr. Hunt remarked that the granites on Marble- 



i 



