BY THE CONDITIONS OF THEIR EXTRUSION. H 



with an indefinite downward extension, filled either with vein or dyke stones, according to 

 the conditions of the rocks in which they occur. It is perfectly clear that the fissures in 

 which the vein stones occur often existed as gaping fissures before the deposits they 

 hold were introduced into them, and we are justified in believing that the fissures con- 

 taining the dyke stones were formed in the same way. As before remarked, vein stones 

 are characteristic of the higher lying and less metamorphic rocks. Though they are 

 found among the rocks containing dykes, they occur there, it seems to me, as deposits 

 made at another period than that which gave birth to the dykes. 



There seems to me no good reason to suspect that the furrows containing dykes have been 

 to any extent riven by the injection of the dyke, as is assumed by many writers. It is 

 likely that the inrushing of materials having the weight and fluidity that belonged to the 

 molten lava would have exercised a certain rending effect upon the rocks in which the 

 dyke was formed, but there are abundant reasons for disbelieving that the fissure itself 

 could have been originally riven by the actual force of the injection. 



The hypothesis of the derivation of these lavas from the more siliceous beds of 

 the subjacent section is a less easily demonstrable part of our proposition. It rests 

 upon the following classes of evidence. In the dyke stones of any district we usually 

 find a very decided difference in composition among the several classes of injections that 

 are found there. In some cases, dykes of one well distinguished class can be found 

 in one set of beds, and yet not appear in those of a lower kind. This class of facts has 

 not been made the subject of careful study by our geologists, yet from my own 

 observation I am satisfied of the truth of this assertion. No one can observe with 

 care the distribution of the dyke stones of New England without being convinced 

 of the truth of this proposition. The rocks on the shores of Maine and Massachusetts 

 give some distinct examples of this class of facts. The dykes of peculiar felsite 

 porphyries of the Marblehead district are not found below the level of the stratified 

 deposits of this age ; i. e., they do not exist over the older jjarts of the field wherein they 

 lie. The amygdaloids, with porphyry deposits contained in the beds, are not found 

 beyond the area of the conglomerates of the Cambrian age. The great series of highly 

 metamorphosed slates and shales like York Harbor and Bald Head on the coast of Maine, 

 contain a wonderful set of melaphyre dykes which are not found in the subjacent grani- 

 toid rocks, though there is good evidence that these underlying rocks have not been much 

 changed since the series of slates were formed. Basalts, ejections of the general mas- 

 sive character of those which make up the Palisades of the Hudson, and the extensive 

 dykes of the Connecticut Valley, are not found beyond the limits of the rocks of Triassic 

 age in those districts. Though the same general classes of basalts are found in other 

 regions near by, they never take on the peculiar facies which they have in these 

 districts. While many of these peculiarities in the distribution of injected rocks may be 

 explained in other ways, I am strongly inclined to believe that they cannot generally be 

 explained, save by the hypothesis that they are dependent on the peculiarities of the sec- 

 tions in which they are found. That is to say, they are formed from the deeper lying 

 highly metamorphosed rocks of each district, and cai-ry the peculiar stamp which is 

 thereby imposed upon them. I do not mean to deny that many of the dyke stones 



