222 RECORD OF HCIENCE FOR 1886. 



roxene. The bornblendic peridotite is dotted with small rounded grains 

 of olivine or serpentine, as in the Schillerspath or bastit of the Hartz 

 Mountains, and for which the name poicilitic (mottled) is proposed. 

 The hornblende is of the variety called basaltic, and contains charac- 

 teristic specular intrusions. It is not crystalline, but fills spaces of the 

 rock, and often forms masses of some size, the other minerals appear- 

 ing to have crystallized from it, probably under much pressure. The 

 olivine has interesting inclusions, and at its contact with the feldspar 

 it is always separated by a zone of square grains of pyroxene and tufts 

 of radiated actinolite. The pyroxene generally appears to be hyper- 

 sthene. Feldspar is never an important constituent. This rock grades 

 into augite-peridotite by transition, the hornblende giving place to 

 augite. The rocks are mostly massive, but are sometimes somewhat 

 schistose, and appear under the microscope to have been subjected to 

 much pressure. All the basic members of the Cortlandt series are too 

 acid to pass as representative olivine rocks, as the amount of this con- 

 stituent is small.* 



97. The petrography of the peridotite in the Carboniferous of eastern 

 Kentucky is described by Diller, who finds it to consist in the main of 

 about 40 per cent, of olivine, 30 of secondary serpentine, 8 of py- 

 rope, and 14 of dolomite. The relation of the peridotites to the asso- 

 ciated sedentary beds is discussed, and as the latter are indurated, and 

 its fragments occur in the peridotite, it is thought that the latter is 

 undoubtedly intrusive.t 



98. Lindgren describes the petrographic features and the relations 

 of the eruptive rocks of south central Montana, as a supplement to 

 Davis' paper (see 85). It is found that the oldest eruptive is a gran- 

 ite with red orthoclase, occurring in dikes in gneiss at the head of 

 Belt Creek, in the Little Belt Mountains. The next period of erup- 

 tion appears to have been in the Cambrian or the Silurian, and rocks 

 consist of a diabase, sometimes quartzose, with the quartz granopyric 

 in structure. This was succeeded in the Jurassic by acid magmas 

 issuing as flows and dikes of diorite, granites and quartz porphyrites, 

 and latter presenting two varieties, the augitic and the hornblendic. 

 In the Laramie and Tertiary, volcanism was very active and varied. 

 The sequence of eruptions is not worked out, but the andesite flows 

 ai^pear to have commenced late in the Cretaceous or Laramie. The 

 hornblende dacites are found massive in the Little Belt Mountains, as 

 dikes in Laramie in front of the main range, and with andesites in the 

 Laramie conglomerates in the High wood Mountains. No augite-da- 

 cites, and only a few augite-andesites were found. Liparites are found 

 in front of the main range as dikes in Laramie strata. The most recent 

 eruptions appear to have been in the post-Tertiary, probably Pliocene, 

 and consist of trachytes and basalt. The former are nearly all devel- 



* Am. Jour. Sci., iii, vol. 31, pp. 26-41. 

 Ubid., vol. 32, pp. 121-125. 



