Position of Porphyry. 449 



been protruded through or among the stratified rocks, subse- 

 quent to their deposition. I say protruded: for if there be ground 

 for making any theoretical inferences whatever in geology, it 

 seems to me that it exists in this instance. The , mere exist- 

 ence of these rocks, therefore, among those of any particular 

 stratified class, does not prove that they were produced at the same 

 epoch : it rather proves that the unstratified rock was of subsequent 

 production. On the other hand, if we find a stratified rock lying 

 above others Avhich are penetrated by veins of an unstratified rock, 

 while this superior one is never thus penetrated, we may safely infer 

 that it was deposited since- the protusion of the unstratified rock. 

 These seem to be the two grand limits of our inquiries in respect to 

 the ages of the unstratified rocks. And if this were all that is meant 

 when the inquiry is whether they are primitive, transition, or secon- 

 dary, a satisfactory answer might be given. In respect to the relative 

 position of the porphyry of Massachusetts, however, I have but little 

 to say, because but few facts have fallen under my observation. I 

 have never met with an instance in which this porphyry was exhib- 

 ited in juxtaposition with any stratified rock : except as already re- 

 marked, the compact feldspar succeeds to the graywacke as an older 

 rock and gradually passes into porphyry. This porphyry, however, 

 is associated, both on the north and south of Boston, with sienite; and 

 in all cases, so far as I have observed, the porphyry lies above the sien- 

 ite, and there is a gradual transition between the two rocks. This 

 fact is most obvious in the Blue Hill range, where one is often 

 much perplexed to decide whether the rock be sienite or porphyry. 

 The sienite in these cases, however, it is important to remark, is never 

 so far as I know, that variety consisting of compact feldspar and horn- 

 blende, which occurs as a member of the overlying family of rocks, 

 but that variety composed essentially of feldspar, quartz and horn- 

 blende, which is connected with granite. Hence I infer that our por- 

 phyry belongs to the oldest varieties of this rock that have been 

 described. 



Mineral Contents. 



Although in South America, according to Humboldt,porphyry forms 

 the matrix of gold and some other metals, yet in general this rock is 

 remarkably destitute of foreign minerals. It is so in Massachusetts. 

 In Maiden it contains a little specular oxide of iron, and this is the 

 only mineral hitherto announced as occurring in it. A careful 

 57 



