" 179 



silicate rocks is handed down from one formation to another. 

 Like begets like in geology as well as in biology. The off- 

 spring always resembles the parent, with certain modifications ; 

 the likeness being strongest with the more important characters. 



If the Montalban gneisses and mica slates had contributed 

 appreciably to the formation of any pai-t of the Shawmut 

 group, a marked difference in chemical composition and crys- 

 talline character would, of course, be apparent in the latter. 

 Now the Taconian series of Western New England — the 

 quartzite, hydro-mica slate, and argillite of which have prob- 

 ably been derived from the underlying Montalban of the Green 

 Mountain range — appears, in the opinion of many geologists, 

 to occupy about the same intermediate position between the 

 Eozoic and Paleozoic that I have assigned to the Shawmut 

 group ; and I would suggest that the chemical and mineralogical 

 contrasts presented by the two series are not greater than their 

 unlike origins would lead us to expect. 



Geologists are now pretty well agreed that to this same 

 transition period belong the copper-bearing beds of Lake Supe- 

 rior ; and Dr. Hunt appears recently to have correlated these 

 provisionally with the Taconian of the Appalachian region.^ 

 Here, again, the underlying crystalline series, for the most part 

 at least, is the Huronian ; and it is in the highest degree inter- 

 esting to observe that the derived series has the same twofold 

 aspect, and, omitting the copper, presents the same general 

 chemical and mineralogical characters as in Eastern Massachu- 

 setts. In both districts we have petrosilex breccias and sand- 

 rocks forming an acidic group, and a basic group composed 

 of chloritic and epidotic, slaty and amygdaloidal beds. 



I have but little hesitation in referring to about the same 

 horizon as the Shawm^t group the hard, argillaceous, silicious, 

 chloritic, and serpentinic slates (the "flinty slate " of Hitchcock, 

 by whom it is well described, p. 550), and the associated do- 

 lomite, that come between the Montalban granite and Carbonif- 

 erous argillite on the peninsula of Newport, R.I. ; the more 



> Chemical and Geological Essays, preface to the second edition. 



