LITHOLOGY. 171 
at which their materials are fusible. These views indicate that not only 
can eruptive rocks be derived from the unsolidified molten materials 
that are beneath the earth’s crust, but that matters once solidified may 
be refused, and that sedimentary deposits impregnated with water, 
though lying far removed from the seat of pure, igneous fusion, may be 
melted, and be erupted if circumstances favor. 
Dr. Hunt, who has advocated these views in this country, has been 
led by his studies to believe that the crust of the earth is so thick, and 
the agencies for liquefying once consolidated matter so efficient, that it is 
not to be expected that any erupted matter comes from such depths as 
that occupied by material which has never been consolidated, and that 
we are to look among the sediments for the equivalents of all erupted 
rocks, This view has been opposed by Prof. Dana, who points to the 
almost perfect uniformity in the composition of the Mesozoic traps, from 
Nova Scotia along the whole of the eastern border of our continent ; 
and which could not be expected in masses of fused sediments. It is not 
my place to discuss these theories. I only wish to make it plain, that if 
some clefts do descend to the matter which represents the original crust 
of the earth, it is admitted by all that sedimentary deposits may become 
fused or plastic; and in this circumstance we have an explanation of the 
fact that many of our eruptive rocks are very basic, many others are 
less so, and many more, which we are going on to consider, are highly 
acidic. Now, the Mesozoic diabase rocks which Prof. Dana shows to be 
so uniform in composition, form immense ridges, and occupy large fis- 
sures which were made by a wide-spread general subsidence, which 
would very likely produce profound fractures. In New Hampshire, no 
such uniformity of condition can be pointed to. Here and there, all over 
the state, are big and little cracks, often so small that they could scarce 
be expected to be very profound, and they are filled with basic rocks, the 
diversities of which indicate eruption at different times; and if, as will 
be shown, dykes of the same form and appearance are filled with acidic 
rocks, then diversity in the original composition of the basic eruptive 
rocks is certainly to be expected. 
= number of causes seem to have operated to produce structural 
differences. Rocks erupted in the later geological periods usually show 
a variety of effects which the laws:of cooling bodies explain. For exam- 
