198 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
by which the formation is cut may represent the upward extension 
of such intrusions, since the latter are commonly believed to be attended 
by silicated solutions. 
The most serious difficulty in the assumption of Cambrian age is 
the question of structure. Two hypotheses may be suggested. First, 
the pinched syncline at College Hill, the apparently overturned syn- 
cline at Malden, and the overthrusting shown in the vicinity of Chest- 
nut Hill suggest that the strata from that point northward may be 
inverted and that the older Cambrian strata may thus be brought 
to the surface. A second hypothesis is that a fault with upthrow to- 
ward the north occurs along the northern border of the conglomerate 
area from Brighton toward Auburndale. Such a fault could easily 
escape recognition on account of the relative weakness of the rocks 
on either side and the heavy drift covering. Evidences of such a 
fault are not entirely wanting. The long conglomerate ledge by the 
railroad track at West Merten shows intense shearing; the pebbles 
of the conglomerate are elongated and cracked and the front of the 
ledge rises in a straight and smooth escarpment suggestive of faulting. 
At Auburndale, half a mile east of the railroad station and a few rods 
south of the tracks, conglomerate with interbedded slate is found dip- 
ping 85° SE, and the pebbl es are highly sheared. Such a dip is 
unusual along the northern border of the conglomerate and suggests 
drag. On the supposition that the slates are upthrown by faulting; 
the northern crystallines must be assumed to be down-faulted along 
the Malden and Waltham escarpment so that the slates are brought 
into opposition with the Medford conglomerates. 
As regards the first hypothesis it may be said that the strata in the 
district north of the Chestnut Hill fault show no definite evidence of 
inversion. On the contrary, the ripple-marked sandstones in Brighton 
and the apparent conformable passage of the conglomerate upwe zard 
into slate tend to show that the strata are in their normal attitude, 
though it may be said that an unconformable contact might be con- 
cealed by the drift north of the conglomerate boundary. In the second 
hypothesis the evidence of faulting now at hand seems to favor the 
downthrow rather than the upthrow of the slates, for in both the 
northern escarpment of the crystallines and the conglomerate escarp- 
ment along the railroad at West Newton the dip of the suppose 
fault planes, so far as it can be determined, is toward the slate, making 
the latter appear as a down-faulted block. The Auburndale ledges 
lie just about on the strike of the supposed fault at West Newton, oF 
perhaps a little to the north of it, so that the apparent drag indicated 
