2l6 HOLLAND: CHARNOCKITE SERIES. 



senses. The fact that the word Schliere (or its plural Schlieren for 

 which form our Anglo-Saxon plurals should prepare us) is a foreign 

 word, will contribute to the preservation of its precise technical mean- 

 ing, and I would, therefore, propose that it be used in the sense in 

 which it is used in Germany. 1 Schlieren may thus be defined, as any 

 portions of a great eruptive mass which show a definite departure, 

 either structural or mineralogical, from the average rock, or main 

 mass ; but which nevertheless are connected with, and show their gene- 

 tic relationship to, the main rock-mass by gradual passage forms, 

 Schlieren are not sharply marked off from the rock in which they 

 occur like inclusions of foreign rocks generally are, but, on the 

 contrary, show under the microscope an interlocking of the consti- 

 tuents across the junction line. Schlieren phenomena may be due to 

 concentration of any portion of the constituents, as in the case of the 

 basic highly hornblendic patches so common in syenites, and the ultra- 

 acid so-called " contemporaneous veins " in the granites ; or they 

 may be due to structural departures from the average rock, as for 

 example the glomero-porphyritic patches in some dolerites. In other 

 words, they may be due to variations in composition or variations in 

 structure. These statements are quite independent of any theories 

 as to the mode of formation of schlieren ; whether they are the result 

 of original heterogeneity of the magma, or due to subsequent segrega- 

 tive consolidation, is of no immediate concern to us ; the important 

 point for the present is the recognition of the features which show the 

 genetic relationship of the schlieren to the rock in which they are 

 found, and the distinction between them and included fragments of 

 foreign rocks which have been picked up by accident. Both features, 



1 " Eine Schliere ist eine Partie eines Kdrpers, welche von der ubrigen Masse differirt, mit 

 derselben jedoch durch Uebergange verbunden ist " (Reyer, Theoretische Geologie, p. 81). 

 " Mit dem Namen Schlieren bezeichnet man die Erscheinung, dass in einer grosseren Eruptiv- 

 masse untergeordnete Partien vorkommen, welche mineralcgisch oder structurell betrachtlich 

 von der Hauptmasse abvveichen, aber mit ihr durch Uebergange verbunden sind . . Da 

 sie keine scharfen Grenzen zeigen, sondern ganz allmahlich in die Hauptmasse verlaufen, so 

 ergeben sie sich als integrirende Theile der letzteren und dvirfen somit durchaus nicht mit 

 frerr.den eingeschlossenen Bruchstucken verwechselt werden ; ihre Bildung hangt auch mit 

 derjenigen der Gesteinsmasse, in welcher sie vorkommen, unmittelbar und untrennbar 

 Zusammen " (Zirkel, Petrographie, 1893, I, p. 787), 



( 98 ) 



