MUSEUM OF COMI'AUATIVE ZOOLOGY. 175 



of the felsites does not appear to have a very extensive development 

 in that region. 



Within the felsites many junctions of apparently different rocks 

 may he seeti ; but because, as it appears to us, it may be possible for dif- 

 ferent parts of the same eruptive mass to form distinct junctions with 

 each other, we were not inclined to accept junctions alone as evidence 

 of difference in ago unless supported by other facts. It appears to be 

 evident, however, that not only at Marblehead Neck, as shown by Dr. 

 Wadsworth, but also in each of the two areas of felsite we have explored, 

 there were at least two distinct eruptions of felsite. 



That some of the felsites are distinct in age is clearly shown also by 

 their relations to the fragmental rocks, the so-called V)reccias, with which 

 they are associated ; but these relations can be considered to better ad- 

 vantage after the latter rocks have been described. 



Fragmental Rocks. 



The complex group of rocks included under the above name embraces 

 those commonly called breccias in this vicinity, and is composed of 

 members wholly distinct in origin and composition. The coarse frag- 

 mental rocks, whose fragments may be either angular or well rounded, 

 are perhaps more abundant than the sandstone and finer-grained rocks. 

 Most of these rocks are composed of fragments of highly altered igneous 

 rocks, the felsites and granites, and properly belong to the tufas, or, accord- 

 ing to Dr. Wadsworth's classification,* the porodites. There are, however, 

 a number of localities where the conglomerate is composed chiefly of 

 pebbles of quartzite from the stratified group. 



The volcanic ashes to which we have already referred as occurring in 

 the neighborhood of Break-heart Hill are undoubtedly of igneous origin, 

 but in other regions the material is distinctly stratified, and must have 

 been produced by aqueous agencies. Although we have no evidence 

 which proves certainly the relative age of the ashes, it seems probable, 

 from the fact that they are so intimately mixed with the oldest felsite, 

 that they were produced about the same time with the felsite, and earlier 

 than the stratified porodites (tufas) and conglomerates. It is sometimes 

 extremely difficult to distinguish in the field between a recomposed rock 

 and an eruptive one which has, at the time of its extrusion, picked up 

 many fragments.t Only those exposures which are quite certainly of 



* Bulletin of the Mas. Comp. Zool., Cambridge, Mass., Vol. V., No. 13, p. 2S0. 

 + See paper by the writer. Proc. B. S. X. H., Vol. XX. 355. 



