LOCATION OF THE TRIASSIC VOLCANOES. 421 



tion towards their hypothetical volcanic centers of eruption ; but the inter- 

 pretation here suggested is very satisfying when on the ground with the 

 whole district spread out before the observer, as is illustrated in figure 2. 



Let the observer climb to the crest of Higby mountain or to any of the 

 other summits near Meriden. It is manifest from these fine points of ob- 

 servation that the lava sheet of which the ridge is formed once continued 

 upward on the plane of its dip into the air, as it still continues downward 

 under ground. The main sheet may be traced forty or fifty miles along its 

 outcrop ; it may reasonably be supposed to have had a breadth of a quarter 

 or half of this measure. Its great volume suggests a vent of good size from 

 which the lavas were poured out. The association of ashes and lava blocks 

 with the anterior sheet indicates violent explosive action and the building 

 of volcanic cones at the center of eruption. Not a trace of these cones can 

 now be found. They may still be buried in the lower part of the eastern 

 half of the monocline; but we have no means of testing this supposition. 

 They may have been eroded from the uplifted part of the western half of 

 the monocline; and this supposition is strongly supported by the occurrence 

 of the great irregular " neck " or network of dikes in Mount Carmel. There 

 is no demonstration of connection between these volcanic roots and the out- 

 flowering surface sheets ; but the two parts correspond so w^ell that the sup- 

 position of their genetic connection is eminently satisfactory, even though it 

 involves the wholesale destruction of the uplifted volcanoes in Jurassic time. 



Objection may of course be made on the ground of the great erosion that 

 this supposition involves. Certainly a great amount of material has been 

 denuded if the section is here drawn correctly ; but, in spite of that, this is 

 still the best interpretation that I can offer. 



Verification of Inferences as to Structure. 



It is manifest that the correctness of the interpretation here briefly sketched 

 depends in large measure on the certainty with which the faults in the for- 

 mation are demonstrated. I have therefore given particular attention to 

 the evidence on Avhich their recognition depends, and have made it the main 

 theme of the exercises in the seven-day halt of the Harvard summer school 

 in the Meriden district, as mentioned above. It seems to me to reach an 

 absolute demonstration. 



The character of the evidence is as follows: In the first place, an exami- 

 nation of the lava sheets of the district shows them to be extrusive, because 

 they are vesicular and slaggy at the upper surfaces ; because they are asso- 

 ciated with beds of ashes; because their fragments, large and small and more 

 or less water-worn, occur in the overlying sandstone ; and because the bedding 

 of the overlying sandstones conforms to the inequalities of the lava sheet, 

 even filling the small crevices and open vesicles at the surface. Under the 



