180 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
in this case may doubtless be explained as an accompaniment of the 
intrusion of the neighboring melaphyr. Other instances of ripple- 
marking may be seen along the railroad east of Newton Center (Boston 
III, V 2), in the ledges on the south side of Forest Hills Cemetery 
(Boston VI, R 18) and at Milton Lower Mills (Boston VI, XY, 25-26). ` 
Ripple-marks as well as cross-bedding seem to be generally distrib- 
uted rather than confined to any particular region, and to form only 
subordinate features of the ledges in which they occur. Mud-cracks 
have been seen in only one locality, the ledge at Forest Hills Cemetery 
just mentioned. No raindrop impressions, footprints or other organic 
markings on the surfaces of the rock layers have been observed. It 
may be said, however, that vague trails and impressions occur inthe 
Somerville and Neponset slates and have been described by Wood- 
worth (a, p. 127-128). Burr’s discovery of indefinite casts of tree- 
trunks in the sandstone and grit of the Forest Hills locality has been 
mentioned on page 100. One such form was seen there by the writer. 
——:— Local Unconformities. No actual examples of local uncon- 
formity have been observed in the Boston Basin but there is some 
evidence of contemporaneous erosion of the sediments during their 
deposition. Pebbles of conglomerate, grit, and sandstone, of the same 
composition and texture as the corresponding rocks of the conglomer- 
ate series, occur in the conglomerate at several places. Eleven such 
instances have been noted, as follows: Medford (Boston IV, N 31-32), 
West Newton (Boston II, K 29 and G 35), Newton Center (Boston III, 
U 1-2), Brighton (Boston V, H 26), Brookline (Boston VI, A 6 and © 
1), Jamaica Plain (Boston VI, L 9 and M 14), Hyde Park (Boston VI, 
L 30-31), and Hough’s Neck (Boston Bay III, J 30). As in the case 
of cross-bedding and ripple-marks the contemporaneous erosion is 
not limited to particular areas but appears to have occurred generally 
throughout the basin, though not on an extensive scale. Certainly 
such contemporaneous erosion must mean local unconformities some- 
where. Perhaps more diligent examination of existing outcrops or 
future exposure of ledges now covered may bring some examples to 
light. 
Relations to Subjacent Rocks. At several places in the Boston 
Basin the relations of the conglomerate series to the subjacent rocks 
are clearly shown. In Medford, according to LaForge, the conglom- 
erate grades into arkose which rests against its parent granite (La Forge 
p- 89). Similar phenomena are seen at East Dedham. In Hyde 
Park and South Natick the conglomerate becomes increasingly felsitic 
