474 SCIENCE PROGRESS. 



finding fresher specimens in which some leucite still re- 

 mains. 



The presence of melilite in certain dykes in the region 

 is another point of interest. Smyth in 1892 described a 

 rock from Manheim, N.Y., under the name peridotite, as 

 consisting of abundant olivine, biotite, magnetite, and 

 perofskite, with considerable quantities of alteration-pro- 

 ducts. Later study of fresher material has enabled him to 

 identify melilite as a constituent, and the rock becomes an 

 alnoite or melilite-basalt (25). The melilite shows the char- 

 acteristic "peg-structure," but differs optically from the 

 mineral as usually known in having positive double re- 

 fraction. An alnoite had already been described by Adams 

 from Ste Anne de Bellevue near Montreal (26). Here the 

 porphyritic elements are large crystals of brown mica 

 (anomite), olivine (with conversion to haematite), and augite, 

 and the ground-mass consists of the same minerals with 

 melilite, magnetite, apatite, and perofskite, the rock agreeing 

 very closely with that of Alno, off the coast of Sweden. 



Some other porphyritic dyke-rocks which have been 

 called peridotites might perhaps be placed with more 

 propriety under monchiquite as olivine-bearing lampro- 

 phyres. Darton and Kemp (27) have described such a 

 rock from De Witt, near Syracuse, in the same district as 

 the Manheim alnoite. It contains abundant olivine and 

 porphyritic crystals of biotite and subordinate augite in an 

 augitic ground-mass of the monchiquite type. The peri- 

 dotite of Pike County, Arkansas, seems to be closely similar. 

 Of the monchiquites and the typical fourchites and ouachit- 

 ites of the last-named state it is not necessary to speak — 

 we have referred to the work of the late J. F. Williams on 

 a former occasion. Here, as elsewhere, these peculiar 

 lamprophyre dykes figure as the satellites of plutonic rocks 

 of the nepheline-syenite family, though some occur at 

 considerable distances from any visible outcrop of the 

 latter rocks. These Arkansas intrusions are assigned to an 

 epoch about the close of the Cretaceous period. 



We pass westward in our hasty survey to the Rocky 

 Mountain region. Limitations of space compel us to omit 



