LITHOLOGY. I/I 



at which their materials arc 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 re-fused, 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. 



A 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- 



