60 GEOLOGY OF TEE NAEEAGA]S:SETT BASIN. 



when examined, is seen to go very far, and in several directions. In the 

 first place, the conglomerate may be taken as representing the beds which 

 were exposed to erosion at the time it was formed. If the beds are of 

 ancient and of known age, thej-^ may enable ns to determine the former 

 existence, in the field, of rocks which have since disappeared by erosion, 

 been lowered beneath the sea level, or been covered over by other deposits. 

 Thus, in the case of the Carboniferous conglomerates of the field mider 

 consideration, we find in the beds a very great number of quartzitic pebbles 

 which contain fossils evidently of the Cambrian age. It is clear that the 

 field occupied by the quartzites was extensive, for the fragments which 

 appear to be of that gronp of rocks, though not always containing fossils, are 

 abont the most numerous of the components which make up some of the 

 thickest layers of the Carboniferous conglomerates. A carefnl search of the 

 rocks of eastern Massachusetts has failed to reveal the sonrce of these fossil- 

 bearing pebbles. Strata perhaps about the same age are found in varions 

 parts of eastern Massachusetts, but they are lithologically and in fossil con- 

 tents very difi^erent from the strata which afforded the pebbles. While it is 

 possible that the field whence the quartzite bits came has, by differential 

 warping, been carried beneath the sea» it is rather improbable that such has 

 been the case; it is more likely that the rocks in question lay on the margin 

 of the basin, whence by erosion they have disappeared. 



It is a noteworthy fact that the above-mentioned quartzite is the only 

 rock of the many contained in the Carboniferous conglomerates which has 

 disappeared from this part of the country since these beds were formed. 

 There are, it is true, certain uncharacteristic sandstone pebbles which 

 can not clearly be identified with anything now in or about the basin, but 

 these are not numerous. The impression left by the study of the Coal 

 Measures pebbles is that the general character of the rocks exposed at the 

 surface in this field in Carboniferous time was substantially the same as 

 that of the rocks remaining at the present time. This view is justified by a 

 comparison of the materials contained in the ancient and the modern aggre- 

 gations of glacial waste. Taking pains to exclude from the waste of the 

 last Glacial period the pebbles which have been worn from the basin rocks 

 of this field, the observer is at once struck with the likeness of the two 

 assemblages, a likeness which shows us that the erosive agents found, with 

 the exception of the above-noted quartzites, much the same rocks open to 



