4 66 SCIENCE PROGRESS. 



chief mineralogical feature being the coming in of abundant 

 little grains of sphene, which indeed seems to point to a 

 local enrichment in lime. On the western shores of Lake 

 Champlain, again, Kemp (4) describes the limestones as 

 becoming coarsely crystalline near the gabbro and charged 

 with bunches of various silicates and other minerals : 

 quartz, plagioclase felspar, diopside, hornblende, scapolite, 

 brown mica, pyrrhotite, tourmaline, sphene, etc. Scapolite 

 is characteristic in this connection throughout the region, 

 and is always accompanied by pyroxene and hornblende. 

 Further details are given in a later paper (9). The crystal- 

 line limestones are associated with various gneissic and 

 schistose rocks, doubtless also metamorphosed sediments, 

 and in the upper part of the series come serpentinous lime- 

 stones or ophicalcites. In the best sections, at Port Henry, 

 the bunches or patches of silicate-minerals, etc., enclosed in 

 the limestone range up to masses twenty-five to fifty feet 

 thick. They consist generally of a coarsely crystalline 

 aggregate of plagioclase, quartz, and hornblende, with 

 various other minerals as mentioned above. The serpentine 

 in the ophicalcites, as Merrill had already shown, has been 

 mainly derived from pyroxene, but Kemp finds evidence 

 that garnet has also furnished a part. The rocks also con- 

 tain lenticular patches composed of pyroxene, hornblende, 

 sphene, and phlogopite. These serpentinous rocks occur 

 in the limestones on the west as well as on the east side of 

 the Adirondacks. 



Much attention has been given in recent years to the 

 various igneous rock-types rich in alkalies which occur in 

 the region east of the continental water-shed. A number 

 of interesting examples have been described by Weed and 

 Pirsson from the State of Montana. Two types of phono- 

 litic dyke-rocks were first discovered as boulders, but 

 subsequently traced to their sources in the Bear-Paw 

 Mountains (10). One, styled pseudo-leucite sodalite 

 tinguaite, is of interest especially as containing the pseudo- 

 morphs of orthoclase and nepheline after leucite already 

 known in Brazil, Arkansas, etc., while fresh sodalite is also 



