Wadsworth.] 236 [May 16, 



All previous writers upon the rocks in the localities mentioned, 

 who have undertaken to give their composition, have, as far as I 

 know, considered them all to be hornblendic, except in the ^ase of 

 the Doctors Dana, who in their paper on the " Mineralogy and Geol- 

 ogy of Boston and its Vicinity," on page 157, state that basalt occurs 

 in beds in argillite at Charlestown, and in rounded masses at Cam- 

 bridge and Charlestown, basalt at that time being considered to be a 

 simple mineral. 



In renaming these rocks I shall in a great measure follow the views 

 so ably advocated by Mr. S. Allport in the Geological Magazine, and 

 in the Quarterly Journal of the Geological Society, that intrusive 

 rocks of the same composition should not be given different names 

 according; as they are considered to be older than the Tertiary age 

 or not, and that as the chloritic material is solely a product of altera- 

 tion it should not be made a fundamental basis for their classification. 



These rocks answer to the common definitions of diabase and 

 melaphyr, if they are older than the Tertiary, but rejecting the limi- 

 tation of age (and in this case it is unknown at present), they should 

 be classed as dolerites and basalts, the difference being merely the 

 more compact state of the last, and employing the term dolerite, as 

 suggested by Mr. Allport, as the generic name, retaining, if desirable, 

 the terms diabase and melaphyr to simply indicate altered forms of 

 dolerite and basalt. 



The results of the examination of these rocks will be found similar 

 to those obtained by Messrs. Allport, J. A. Phillips, E. S. Dana, 

 Hawes and others, in their examinations of similar rocks. 



These rocks offer a fruitful field for accurate chemical analysis, 

 and for more extended microscopic observations; from what exam- 

 inations I have been able to make outside of the field chosen for this 

 paper, and from specimens in the collections of others, there is but 

 little doubt that these rocks are very wide-spread in Eastern Massa- 

 chusetts, and that a more extended study of them, and of the other 

 intrusives in this region, would aid much in simplifying its complex 

 geology. 



I would like to urge here that rocks should be most carefully 

 examined in the field as well as in the cabinet, and, when possible, by 

 the same observer, for even in this limited region, and from these 

 dikes, specimens can, and have been collected, as typical ones, that 

 in the cabinet, and even in thin sections, would be, and have been, 



