11 



groups, — the crystallines and the uncrystallines. The former 

 predominate, constituting the surface rocks over fully nine- 

 tenths of the area to which this paper particularly relates. The 

 chronologic distinctness of these groups will probably not pass 

 unquestioned. Yet it is true that, with the exception of the 

 argillites of the Nashua valley, which Prof. C. H. Hitchcock ^ 

 has correlated with the Primordial slates about Boston, none of 

 the crystallines of the eastern part of the State have, of late 

 years, been regarded as of Paleozoic or more recent age, by 

 any geologist familiar with the rocks of this region. 



I have studied the relations of our Primordial strata to the 

 crystallines at every accessible point within my knowledge, and 

 in all cases, where the evidence is of a decisive nature, it affords 

 unequivocal support to the view, that between the crystallines 

 and the oldest uncrystallines there is a great chronologic break, 

 a ' ' lost interval " of immense duration ; for the unconform- 

 ability is perfect, and the imperfect lithologic resemblance 

 sometimes observable is due in every case to derivation or to 

 very local alteration. Furthermore, it will be shown in the 

 sequel, mainly on stratigraphic grounds, that the oldest rocks 

 in the State are on its eastern border, facing the Atlantic, and 

 that the various crystalline series appear to be so arranged 

 geographically as to become successively newer as we proceed 

 from Massachusetts Bay to the Berkshire Hills ; so that, even 

 if there were crystallines of Paleozoic age in Massachusetts, 

 they would not probably be found in the eastern portion of the 

 State. I cannot but regard the distinctness of these two great 

 classes of rocks in this region as the most certain of all the 

 conclusions reached in this paper. Considering the Primordial 

 horizon as the base of the Paleozoic system, then it is true that, 

 in this region at least, the profound break between the crys- 

 tallines and the uncrystallines is coincident with that recognized 

 generally between the two grandest chronologic divisions, the 



1 Geological Map of Massachusetts, in Walling's Atlas, 1871. Subsequently, in the 

 final report upon the Geology of New Hampshire, Prof. Hitchcock has referred these 

 argillites to the ancient Huronian system. 



