42 History of Hingham. 



the slates of the basin must have steadily accumulated through 

 long ages. 



The origin of another abundant rock of the basin, associated 

 with the conglomerate, the melaphyr, long continued to be a ques- 

 tion of much discussion, but there is now no doubt concerning it. 

 During all the immense time that subsidence continued, and 

 while sedimentary strata were gradually accumulating, the area 

 of the basin remained a great centre of igneous action, and vol- 

 canoes here and there within it belched forth from time to time 

 floods of lava which spread itself over the surface. Professor 

 Crosby has made out in the Nantaskct region several flows of it, 

 each of which alternates with deposits of conglomerate and sand- 

 stone. In such cases the outpouring was probably beneath the 

 surface of the water, where the deposits followed each period 

 of activity. In Hingham the melaphyr is found in very great 

 bodies not separated by deposits of the sedimentary rocks. 



One more great event in the history of the basin is yet to be 

 mentioned. Long after the volcanic action tbat had produced the 

 basic lava, melaphyr, bad ceased, and after all the sediments were 

 deposited that produced the rocks known to us as the conglomer- 

 ates, the sandstones, and the slates, a great disturbance occurred 

 over the whole area of the basin and of the crystalline rocks 

 surrounding it, caused by another manifestation of igneous en- 

 ergy, which changed the whole character of the surface. With- 

 in the basin, apparently from immense pressure exerted in north 

 and south directions, the rocky strata were forced up in folds or 

 in broken ridges. Through crystalline rocks and sedimentary 

 strata alike, subterranean action brought to the surface, and 

 probably poured over it, vast quantities of lava of highly basic 

 properties, different from those of the previous eruptions, now 

 known to lithologists as Diabase, an account of which has been 

 given. The great erosion of after ages is undoubtedly the rea- 

 son why the rock Diabase is not found spread over the surface, 

 as well as within the walls of dikes. 



At length the disturbing action ceased, and the earth, which 

 had been shaken from its foundations to its surface, and rent 

 asunder in a thousand localities, once more became quiescent. 

 The effect upon the area of the basin was great, for where the 

 waters had for an immense period spread themselves over the 

 surface, and under which conglomerates and slates had been laid 

 down, dry land appeared. 



How strange to reflect that in these three words is embraced 

 a fact without which all the stupendous events that have been 

 mentioned, occurring over millions of years, would have re- 

 mained entirely unknown to mortal man ; for with the waters 

 covering the basin, where could a trace of its long history have 

 been found ? 



The rocks of the Boston Basin as they present themselves in 

 Hingham will now be noticed. Unfortunately the non-occurrence 



