36 



PLYMOUTH CONGLOMERATE. 



upon the road, a fine opportunity is presented of seeing the quartz pebbles that have not 

 been much flattened on the exposed surfaces, having the aspect of a most decided conglom- 

 erate. Yet if joints cross the rock, or if it be broken across in the direction of the strike, 

 Fl - 17 - the pebbles will, for the most part, appear so 



flattened that they become almost lenticular, 

 or even folia. And if a fracture be made, or 

 a joint occur, in a perpendicular direction, 

 that is in the direction of the dip, the pebbles 

 almost wholly disappear, or rather seem con- 

 verted into the quartz folia of talcose schist. 

 Both these facts are shown on Fig. 17, which 

 was copied from one of the specimens ob- 

 tained at this locality. Looking at one of 

 these edges, we should have no hesitation in 

 referring the rock to a highly quartzosc 

 variety of talcose schist. But looking at the 

 other edge, we should have no doubt that the 

 quartz folia are merely flattened and elongated pebbles. So strange and unexpected a fact 

 leads the geologist to suspect that he may be deceived : but hundreds of specimens force 

 him to the conclusion that he is not mistaken. 



The quartz in this rock, both in Wallingford and Plymouth, is generally white or a 

 light gray, and though sometimes granular, it approaches much nearer the hyaline variety 

 in most instances. It seems to be quite pure silex rather than a silicate. In a few 

 instances we find pebbles of granite, which are also flattened. 



The suggestion has been made that what I regard as pebbles may be concretions. But 

 the following facts seem to me to show this position to be untenable. 1. We have no 

 other examples of concretions formed of hyaline or granular quartz. 2. Silicious concre- 

 tions, such as chalcedony, formed by gelatinous silex, are banded ; but these nodules 

 show no concentric structure. 3. Concretions are never, as these nodules are, drawn out 

 into the folia of schists. 4. Some of them are composed of granite, gneiss, &c., which 

 are certainly never concretions. 5. If these nodules are concretions, so are the pebbles of 

 quartz and granite found loose in modified drift ; or rather, no line of distinction can 

 be drawn between the concretions and the pebbles. 



The thickness of the rock where the pebbles are manifest, is many hundred feet ; indeed 

 it may be much more, as I have not fully explored it. And if, as we suppose, the strata 

 already described in connection with the conglomerate, are only that rock in an advanced 

 state of metamorphism, the original thickness must have been very great. We have 

 ascertained also that the layers with pebbles extend as far south as Ludlow, though 

 nearly converted there into mica schist ; nor can we doubt but it may be traced much 

 farther north and south, perhaps even the whole length of the Green Mountains. At 

 any rate its occurrence on the opposite sides of that mountain at Wallingford and 

 Plymouth at points not much different from east and west of each other, leads to some 

 interesting suggestions and conclusions. The distance between these points is about ten 

 and a half miles. 



