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a rock essentially composed of dark mica, orthoclase and a felspathic 

 matrix. It was introduced into geological literature by VOLTZ in 1828. 

 Kersanton (Kerzanton). This is the name of a small hamlet on 

 the Brest Roads. It is locally applied to several varieties of rock, rich 

 in plagioclase and dark mica and containing carbonates. These rocks are 

 divided by M. BARROIS^ into two groups : kersantites and mica-porphyrites 

 (porphy rites micacees) ; the former being characterized by a granular, 

 the latter by a porphyritic structure. 



Kersantite. This term is sometimes used in M. BARROIS' sense and 

 sometimes made to include both the granular and porphyritic rocks. 



Lamprophyre. This name was introduced by GUMBEL for certain 

 rocks which occur as small dykes (usually running N. & S.) in the 

 Palaeozoic strata of the Fichtelgebirge, Tlmringer Wald and Voigtland. 

 They occur in strata as high as the Culm Measures. They bear a general 

 resemblance to "diabase," but differ from that rock in containing a large 

 amount of dark mica. GUMBEL remarks that this name is intended to 

 include rocks that have been variously referred to as "mica-diabase, 

 minette, kersanton and kersantite." It is interesting to note that 

 GUMBEL'S lamprophyres are. similar in composition, state of preservation, 

 geological age and mode of occurrence to the mica-traps of Cornwall 

 and Devon. ROSENBUSCH has accepted GUMBEL'S term lamprophyre 

 but has somewhat extended its signification. He applies it to rocks 

 which occur as dykes in disturbed regions and which, so far as composi- 

 tion is concerned, have affinities on the one hand with syenites and on 

 the other with diorites. They may be fine-grained, compact or porphyritic 

 in structure. In the porphyritic varieties felspar rarely occurs in the 

 form of large crystals. The porphyritic texture is due to one of the 

 ferro-magnesian minerals. The frequent occurrence of carbonates is also 

 mentioned by ROSENBUSCH as a characteristic. 



The lamprophyre group is then divided by ROSENBUSCH as follows : 

 SYENITIC LAMPROPHYRES. DIORITIC LAMPROPHYRES. 



Minette. Kersantite. 



Vogesite. Camptonite. 



In the syenitic lamprophyres orthoclase is the dominant felspar ; in 

 the dioritic lamprophyres plagioclase is the dominant felspar. In minette 

 and kersantite a dark mica is the dominant fer.ro-inagnesian constituent ; 

 in vogesite and camptonite either augite or hornblende is the dominant 

 ferro-magnesian constituent. It thus appears that there is little or no mineral- 

 ogical or textural difference between vogesite and syenite, or camptonite 

 and diorite. The difference lies in the mode of occurrence. ROSENBUSCH 

 does not refer to the occurrence of any amorphous interstitial matter 

 in his lamprophyres. It seems clear, however, from the researches of 

 BONNEY and others that amorphous matter is present in certain rocks 

 which have precisely the same mode of occurrence as the lamprophyres 

 of ROSENBUSCH. When the dominant felspar is orthoclase BONNEY calls 



(1) Sur le Kerzanton de la Hade de Brest. Ann. d.l. Soc. Geol. du Nord (1886). T. XIV., p. 3. 



