Crosby. j ' 14 [January 36, 



unconformability between different portions of the slate, and dif- 

 ferent portions of the conglomerate. While the general fact 

 remains that adjacent ledges of the two rocks usually show a 

 perfect agreement in strike and dip ; and when the actual contact 

 between the conglomerate and slate is exposed they are always 

 perfectly conformable, except at the ltdge on Beacon Street in 

 Newton Centre ; and here, as I will show later, the unconforma- 

 ble contact is due to irregular faulting and not to erosion. 



4. In the statement that the conglomerate underlies the slate I 

 do not mean to assert that it is older than all the slate ; for it is 

 probable that these rocks are in part contemporaneous deposits, 

 slate in the deeper water and conglomerate in the more shallow, 

 being formed simultaneously. In other words, the deposition of 

 the conglomerate began first, but had not entirely ceased in some 

 parts of the basin, when the deposition of the slate had begun in 

 other parts ; so that chronologically the two deposits overlap, 

 and have not everywhere the same relative thickness. 



The principal facts proving that the conglomerate underlies 

 the slate are : — 



(1) At most points around the margin of the basin the con- 

 glomerate comes between the slate and the crystalline rocks. In 

 many places, it is true, this conglomerate border appears to be 

 wanting ; but the most important of these apparent exceptions 

 occur where the rocks are entirely concealed by drift, and we can 

 not know with certainty what they are. This is the case along 

 nearly the whole of the north side of the basin ; yet toward the 

 southwest, in Needham, and toward the northeast, in Medford, 

 the conglomerate very plainly intervenes. In some of th&se 

 exceptional cases, again, there are indications of faults which, 

 with the downthrow on the side toward the slate, have carried 

 the conglomerate below the present surface. In btill other in- 

 stances the contact of the slate with the bordering crystallines 

 may, w ith considerable probability, be regarded as true overlap 

 resulting from the progressive subsidence during and preceding 

 the deposition of the slate. This explanation seems applicable at 

 some points in the South Shore district. The geographical rela- 

 tion here insisted upon is best shown around the three peninsulas 

 or tongues of the ancient formations (eruptive rocks) which pro- 

 ject into the basin from the west. The isolated masses, or 



