152 Barrell — Growth of Knowledge of Earth Structure. 



rivers. The continued swelling of the stream first overcomes 

 the resistance of its frozen surface and having elevated it to a 

 certain extent, it is forced into a vertical position, or thrown 

 over upon the unbroken stratum behind, by the progressive 

 power of the current. 1 ' 



So far as the present writer is aware this is the first 

 recognition in geological literature of the evidence of a 

 horizontally compressive and overturning force as a 

 cause of folding. 



To E. Hitchcock belongs the credit of being the first to 

 describe overturning and inversion of strata on a large 

 scale, but without clearly recognizing it as such. In 

 western Massachusetts metamorphism is extreme in the 

 lower Paleozoic rocks in the vicinity of the overthrust 

 mass of Archean granite-gneiss which constitutes the 

 Hoosic range. The Paleozoic rocks of the valley to the 

 west are overturned and appear to dip beneath the older 

 rocks. Farther west the metamorphism fades out and 

 the series assumes a normal position. Such an inverted 

 relation, up to that time unknown, is described in 1838 as 

 follows by Hitchcock in his Geology of Massachussetts 

 (pp. 297, 298) : 



"But a singular anomaly in the superposition of the series of 

 rocks above described, presents a great difficulty in this case. 

 The strata of these rocks almost uniformly dip to the east : that 

 is, the newer rocks seem to crop out beneath the older ones; so 

 that the saccharine limestone, associated with gneiss in the east- 

 ern part of the range, seems to occupy the uppermost place in 

 the series. Now as superposition is of more value in determin- 

 ing the relative ages of rocks than their mineral characters, must 

 we not conclude that the rocks, as we go westerly from Hoosac 

 mountain, do in fact belong to older groups ? The petrifactions 

 which some of them contain, and their decidedly fragmentary 

 character, will not allow such a supposition to be indulged for 

 a moment. It is impossible for a geologist to mistake the evi- 

 dence, which he sees at almost every step, that he is passing 

 from older to newer formations, just as soon as he begins to 

 cross the valley of Berkshire towards the west. We are driven 

 then to the alternative of supposing, either that there must be 

 a deception in the apparent outcrop of the newer rocks from 

 beneath the older, or that the whole series of strata has been 

 actually thrown over, so as to bring the newest rocks at the bot- 

 tom. The latter supposition is so improbable that I cannot at 

 present admit it." 



