26 The Ash Bed at Meriden. 



ures in the two cases makes it clear enough that intrusive sheets can 

 be distinguished from extrusive sheets by their physical character- 

 istics, and with a great degree of certainty. In the Meriden dis- 

 tricts, all the sheets can be shown to be of extrusive origin. 



When we picture the conditions of the estuary in which the Tri- 

 assic deposits were accumulated, we must therefore imagine the or- 

 dinary processes of erosion from the adjacent lands and deposition 

 in the waters to have been interrupted occasionally by eruptive action 

 on a large scale. At one time, a sheet of lava whose edge is now 

 visible as the anterior ridge of Lamentation, was poured out over 

 the muddy bottom of the estuary, and accompanying this eruption 

 there was explosive action that gave rise to showers of ashes and 

 bombardment of lava blocks. About the same sort of eruption, 

 but of later date, is known in Massachusetts, on the back of Mt. 

 Holyoke, where it has been carefully studied by Professor Emerson 

 of Amherst. There was then a time of quiet deposition again for 

 a period long enough to form the strata that lie between the ante- 

 rior ridge and the main trap sheet of Lamentation Mountain; and 

 after this, came the great out-pouring of lava to a thickness of sev- 

 eral hundred feet and over an area that can be shown to measure 

 several hundred square miles. No explosive action producing ashes 

 and bombs has been detected in connection with this eruption. 

 Another period of quiet deposition followed, succeeded by a smaller 

 eruption, whose edge is seen posterior to Lamentation in the trap 

 ridge which runs north from Highland Lake. There were probably 

 other eruptions at yet later dates. The intrusion of the West Rock 

 sheet cannot be dated with respect to the eruptions. 



The opportunity for the deposition of the Triassic strata came 

 when the old land surface on which they rest was depressed below 

 water level, so that it received the waste from the adjacent unsub- 

 merged areas. The deposition ceased when a later disturbance ele- 

 vated the submerged area above water level and exposed its accu- 

 mulations to destructive action. 



The character of this disturbing force was very peculiar. It re- 

 sulted first, in breaking up the whole series of deposits, aqueous and 

 igneous together, into relatively narrow blocks from a quarter of a 

 mile to one or two miles or more wide and ten or twenty miles long; 

 and then in dislocating them by tilting them over so that the beds, 

 which were at first horizontal, now incline or dip to the eastward 

 at an angle of about fifteen degrees. The country would have a 

 strange topography if this constructional form had never been 

 changed by erosive forces. A pretty close parallel to it may now 



