1888.] Dr Stecher on Contact-Phenomena. 161 
is, that they belong solely to one group, and this for the following 
reasons : — 
1. Orthoclase does not occur in any notable quantity. The 
“ divergent herring-bone lineation from the plane of twinning,” 
that Professor Geikie* * * § believes to be of orthoclase, was observed 
but rarely, and where found (Hillwood Bath) proved to be the 
weU-known micropegmatitic structure. The species of felspar in 
which the quartz is included could not be satisfactorily determined. 
Excepting these few undecided cases, the felspars represent plagio- 
clase. Although in the heart of the more extensive intrusive sheets, 
this plagioclase always occurs as untwinned crystals, it is never- 
theless a triclinic felspar. It exhibits zonal structure (the so-called 
“ isomorphe Schichtung”), and the extinction angles, measured in the 
most widely separated zones of one and the same individual, vary 
up to 40°. These peculiarities have been shown by Tbrnebohmt 
and HopfnerJ to be characteristic of plagioclase felspar. 
2. As will be showm in a subsequent part of the present paper, 
the quartz grains that occur in these rocks are due to secondary 
influences ; they cannot, therefore, be considered as characteristic. 
However, many of Geikie’s “ dolerites ” {e.g., Hound Point) contain 
in certain zones a considerable quantity of quartz, occurring as an 
authigenic constituent. 
3. With respect to the olivine, what I have just stated with 
reference to the quartz is also true here. All the differences, even 
the most extreme, can be reduced to the influence of the associated 
rocks on the intrusive sheets which have invaded them. 
4. Moreover, we must bear in mind the peculiarities of structure. 
But these were in no case found to prevail throughout the whole 
of a volcanic mass. Neither could I find in the specimens examined 
by me the distinctions pointed out by Professor Eosenbusch, § 
namely, that Geikie^s “ diabase ” has a “ normal hypidiomorphic,” 
and the “ dolerite ” a “ typiscli diabasiscli-hdrnigC or ophitic 
structure. 
* On the Carl. Vole. Rocks, &c., p. 488. 
t Tornebohm, On Sveriges wichtigare diahas och gabhro arter. 
+ Hopfner, “ Ueber das Gestein vom Monte Tajumbina in Peru,” Neucs 
Jahrh.f. Min., 1881, vol. ii. p, 164. 
§ Rosenbusch, M.ikroskopische Physiographie, Aufl. 2, Band. ii. p. 193. 
