1875.] 389 [Dodge. 



Their dip, when it can be distinguished, is almost invariably to the 

 west and north-west, according to their strike. They underlie uncon- 

 formably strata holding Paradoxides, etc., and probably formed hill 

 or island ranges very early in the history of this continent. Some 

 geologists have assumed the presence of a barrier in this neighbor- 

 hood, protecting the inner continental basin from the ocean, to 

 account for the deposition of the Lower Silurian limestones of the 

 States to the westward. 



For the most part, metamorphism has been so complete that these 

 rocks have lost almost entirely their probable original character. 

 Moreover, the igneous outflows have been so numerous and compli- 

 cated that it becomes almost impossible to decide, in some cases, 

 which rocks have been actually fluent, and which were only acted 

 upon by others in that condition, or by the same causes which 

 reduced such others to that condition, in a less degree. On this 

 account, some have considered all the sienite and greenstone of 

 igneous origin. Undoubtedly the igneous and metamorphic varieties 

 are often difficult to distinguish, but inability to separate them is no 

 reason for declaring them identical. While eruptive masses have 

 frequently an appearance of schistose structure, by which this diffi- 

 culty is increased, in metamorphic sienites and diorites, on the 

 other hand, the original stratification is often so completely lost as to 

 be undiscernible; so that, while of sedimentary origin, they may, in 

 view of the probable presence of heat as an attendant on meta- 

 morphic changes, be said, with a certain degree of truth, to be 

 " igneous " rocks. Sedimentary, metamorphic, igneous, — the terms 

 are too little discriminating to be disputed about, and I believe that 

 the question is often merely one of degree of the same action. 

 Metamorphic crystalline rocks usually are cut by veins of the mate- 

 rial of which they are in part composed, in a more fluid state than 

 the pasty body of the rock, often in a manner incompatible with the 

 assignment of percolating solutions as the cause; and overlying rocks 

 often have their cracks filled by intrusion from the more fluid mass 

 below. This is often seen in such more recent rocks lying in the 

 direction of the axis, or in the lateral vicinity of a crystalline range, 

 after denudation. Several instances of this are pointed out below. 



It has seemed necessary to say so much at the risk of falling into 

 geological commonplaces admitted as such by all, on account of the 

 variety of opinions entertained as to the relations of metamorphic 

 and eruptive rocks, and the consequent different significance attached 



