Corresponde::. . 325 
sive material. These supposed dikes are represented as 
transverse to the dip of the enclosing strata ; but their close 
conformity with the strike of the sediments is not explained. 
Another fundamental fact which is overlooked is the oc- 
currence in this area, and in intimate association with the 
melaphyr, of important beds of melaphyr tuff. The genetic 
relations of the tuft' to the melaphyr cannot be questioned ; 
and the existence of the former should be regarded as conclu- 
sive as to the contemporaneous origin of a part at least of 
the latter. Much is made of the supposed absence of mela- 
phyr pebbles in the overlying conglomerates. These are not, 
however, wholly wanting; and at some points they are a very 
striking and significant feature of the contact. For instance, 
between Newton Upper Falls and Newton Highlands, in the 
western part of Mr. Burr's field, a north-south section shows, 
if there be no repetition of the strata by strike faulting, four 
heavy beds of conglomerate and sandstone, all dipping in a 
general northerly direction at angles of twenty to forty de- 
grees, separated by three broad bands of melaphyr. The most 
clearly exposed contact in this series is the uppermost, or that 
Detween the northern band of melaphyr and the overlying con- 
glomerate. This melaphyr is, in the southern (lower) part of 
the band, a solid and homogeneous rock of approximately holo- 
crystalline aspect ; but northward it becomes more slaty, in part 
amygdaloidal. brecciated and scoriaceous : and at last is decid- 
edly shaly in structure, looking- more like a tuff than a flow. 
It is covered by the conglomerate with apparent conformity ; 
and the conglomerate is. on and near the contact, not injected 
by the melaph\r. 1)ut crowded with angular fragments of pre- 
cisely similar melaphyr. Within a yard above the contact the 
melaphyr detritus begins to die out ; and at a distance of two 
yards an occasional fragment only is to be found. The mela- 
phyr in the conglomerate is clearly a contact feature ; and it 
can not be doubted that this body of melaphyr was in existence 
as a surface formation when the deposition of the conglomer- 
ate began. Everything goes to indicate that these effusive 
eruptions were submarine ; and at Nantasket, as also in Brigh- 
ton, we have conclusive proof, in the well preserved wavy and 
ropy surfaces of the flows, that in some instances thev were 
covered without suffering erosion. 
The main bodies of melaphyr, throughout the Boston basin, 
have all the characters of lava flows and are essentially unlike 
any known dikes or sills. In other words they are masses 
which fundamental and indisputable facts indicate to be con- 
temporaneous. And yet, because of certain minor irregular- 
ities of contact and appearance of baking, etc., they are de- 
scribed as intrusive ; and highly improbable sections, with trans- 
verse dikes thousands of feet wide, are constructed to explain 
