G-i BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



manner toward the east, for the conglomerate ridges disappear, and the 

 Chestnut Hill Slate belt apparently becomes continuous with that to 

 the north. 



That faulting of the overthrust type is to be looked for in the region 

 is shown by the occurrence of a second thrust crossing Commonwealth 

 Avenue at Summit Street, half a mile north of the one described 

 (Plate 2, Loc. 22). This is well shown in exposures on both sides of 

 the avenue. The slate is here repeated, lying on top of the conglomer- 

 ate and again overthrust by conglomerate to the north. The outcrop 

 on the east side of the street shows particularly well the way in which 

 the conglomerate has been crumpled by dragging on the plane of the 

 thrust. This fault is seemingly not traceable westward. Toward the 

 east it is complicated by a normal fault of later date. 



The discontinuity of the Chestnut Hill slate belt, the actual occur- 

 rence, in two instances, of faulting on the contact between the slate and 

 the conglomerate, the general discordance in structure between the two 

 rocks and the known occurrence of a fault bringing about similar rela- 

 tions a short distance to the noi'th, all point to the same conclusion, that 

 the conglomerate has been thrust over the slate. All of these facts are 

 very difficult to explain on the supposition that the conglomerate passes 

 in a syncline beneath the slate. 



The stratigraphic succession shown in this conglomerate belt is not 

 such as to indicate anticlinal structure. South of the melaphyr the 

 rocks are prevailingly conglomerate, coarse, as a rule, but becoming 

 finer toward the igneous rock. Near the melaphyr sandstone bands 

 are frequent, and there are occasional interbedded slaty layers. This is 

 the character of the upper portion of the conglomerate in the adjacent 

 Brookline area and quite unlike the massive beds in the lower zones. 

 The sediments in actual contact with the melaphyr are seldom coarser 

 than sandstone. In the eastern extremity they are prevailingly slates. 

 Such rocks are out of place at the base of the sedimentary series, judg- 

 ing from the evidence elsewhere. North of the melaphyr, outcrops are 

 few. The greater number are of slate. Thin conglomerate bands occur 

 in a number of places in association with finer sediments. In two lo- 

 calities, only, are these conspicuous. A bed of conglomerate, perhaps 

 thirty feet in thickness, is exposed on North Beacon Street, Brighton. 

 It conforms in dip and strike to the slates on the north and on the 

 south, and is apparently interbedded with them. Similar beds of con- 

 glomerate are known to occur within the slate series in other parts of 

 the region. Conglomerate is exposed in a cutting near the Boston and 



