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partial interruption of the lithologic gradation between these 

 terranes. There is no room to doubt that the conglomerate is, 

 petrologically, a part of the argillite, being conformably inter- 

 stratified with it ; and farther on I shall be able to show that the 

 conglomerate is, stratigraphically, at the bottom of the argillite, 

 and probably marks the beginning of its deposition. Yet the 

 fact that the conglomerate, so far as we know, reposes conform- 

 ably upon and contains no material derived from the mica slate, 

 allows us to believe that it does not indicate a very prolonged 

 chronological break. These various rocks, of course, are not to 

 be regarded as perfectly conformable, nor as having been de- 

 posited with chronologic continuity. They constitute a vast 

 formation having a volume of many thousands of feet, and 

 probably represent, in their deposition, a lapse of time com- 

 parable with such more modern periods, as the Cambrian, or 

 Silurian, or even perhaps with the whole Paleozoic era. Hence 

 it seems only reasonable to expect to find in portions of the 

 Montalban series indications of more or less irregular and dis- 

 continuous deposition, — stratigraphic and chronologic breaks 

 of magnitudes comparable with those characterizing the newer 

 formations named. 



Unique evidence pointing toward the integrity or unity of 

 this series is afforded by a comparison of the textures of the 

 various rocks composing it. It will have been observed that, 

 in passing from the granite to the gneiss, from the older, in- 

 digenous gneiss to the newer varieties shading into mica slate, 

 from the older to the newer varieties of mica slate, and from 

 these to the argillite (omitting the Harvard conglomerate), we 

 proceed constantly from coarser to finer grained rocks ; the ex- 

 tremely coarse, porphyritic granite and the fine-grained, impal- 

 pable argillite constituting the extremes of a series which is 

 both mechanical and chronological. This formation, viewed as 

 a whole, presents a regular, graded series of textures, which is 

 exactly coincident with the order of sequence of the rocks to 

 which the textures belong. 



That the Montalban rocks of this region, with the exception 



