OP ARTS AND SCIENCES. 255 



VIII. Whenever manageable crystals of feldspar could be got, they 

 were cut parallel to the plane of easiest cleavage, and studied by Des 

 Cloiseaux's method. A modification of Des Cloiseaux's method was 

 used, to aid in determining the feldspar in thin sections of rock cut at 

 random. 



In the Report of the Geological Survey of Michigan, after a study 

 based on the older lithological methods, I distributed the different 

 varieties of these rocks under three heads ; viz., coarse-grained and 

 fine-grained melaphyres, and melaphyre porphyry. In the same 

 volume, my assistant, the late Mr. Marvine, mistaking the augite 

 for hornblende, described the coarser varieties as diorites, which was, 

 at that time, a very common mistake. 



In writing this paper, I had intended to retain my original classifica- 

 tion ; but, at the last moment, I received Rosenbusch's " Mikroscopische 

 Physiographie der massigen Gesteine," in which the plagioclase-augite 

 rocks are distributed in the following manner : — 



I. Pre-Tertiart. 



1. Granular. 



a. Plao;ioclase-au";ite = Diabase. 



h. Plagioclase-augite-chrysolite= Chrysolitic diabase. 



2. Porpkyritic ; containing more or less unindividualized base. 



a. Plagioclase-augite = Diabase porphyrite. 

 h. Plagioclase-augite-chrysolite = Melaphyre. 



II. Tertiary and Post-Tertiary. 

 Granular or Porphyritic. 

 a. Without chrysolite = Augite-andesite. 

 h. With chrysolite = Basalt. 



In thus designating as melaphyre only such rocks as are the older 

 equivalents of those feldspar-augite basalts in which the original fluid 

 magma is more or less represented by unindividualized base, it seems 

 to me that Roseubusch has simplified the troublesome problem involved 

 in assigning to their proper places a large number of partially allied 

 rocks. 



The eruptive rocks of the Keweenaw series, or at least all those 

 treated of in this paper, fall readily under the two heads of Diabase 

 and Melaphyre. 



The greater number of the beds are fine-grained diabase ; coarse- 

 grained diabase is much rarer ; and intermediate between the two, as 

 regards frequency of occurrence, is the melaphyre. 



