532 MESSRS. HILL & KYNAST0N ON KENTALLENITE [Aug. 19OO, 



in the Tyrol. Since then the term has been employed by different 

 authors in slightly varying senses, but in the main as applying to rocks 

 of the Monzoni facies. More recently Prof. Brogger has made a 

 study of these rocks, and has used the term in a far more extended 

 sense. He includes under monzonite a large variety of rocks, with 

 a silica-percentage ranging from 71*42 down to 49*40, in which 

 plagioclase and orthoclase occur in approximately equal proportions. 

 Typical monzonites, according to him, are essentially orthoclase- 

 plagioclase rocks, forming an intermediate group (iibergangs- 

 gruppe) between syenites and granites on the one hand, and 

 diorites and gabbros on the other. It is true that the typical rock of 

 Kentallen might fairly be described as a slightly more basic variety 

 of the Monzoni rock, with the addition of olivine as one of the 

 principal constituents. But in consequence of Brogger's paper, the 

 term has come to be associated with the presence of an approximately 

 equal amount of the two felspars — a feature which cannot be said to 

 be an essential characteristic of our group, while in other respects it 

 is clearly denned. In short, we have in Argyllshire a distinct rock- 

 type, possessing at the same time affinities with the extreme basic 

 variety (olivine-monzonite) of Brogger's group, and showing constant 

 features in widely-separated intrusions. Taking these and other 

 facts into consideration, it has seemed to us advisable to propose a 

 new name for these peculiar Argyllshire rocks. It would, more- 

 over, be unsuitable to employ the term monzonite in the present 

 case in its former signification of augite-syenite, on account of the 

 differences in mineralogical and chemical composition between our 

 type-rock and rocks of the syenite-family. Taking, therefore, the 

 Kentallen rock as our type, we propose that the term kental- 

 lenite should be substituted for olivine-monzonite. Kentallenite 

 may be briefly defined as a coarse or medium-grained noncrystalline 

 rock, consisting of olivine and augite, with orthoclase, plagioclase, 

 and biotite in varying proportions. It is allied not only to the 

 olivine-monzonites (Brogger) of Scandinavia, but also to a peculiar 

 basic rock, associated with syenites in Montana, termed shonkinite 

 by Messrs. W. H. Weed & L. V. Pirsson. These relations will be 

 more fully pointed out in the sequel. 



II. Distribution, Mode of Occurrence, and Description 

 op the Rocks. 



Although the peculiar characters of the kentallenites were not 

 fully recognized at the time, it should be pointed out that they were 

 first mapped by one of us in the Ben Bhuidhe area, about 8 miles 

 north-east of Inveraray, in 1892. In 1896 the rock was again 

 found near Loch Avich, about 9 miles south-west of Kilchrenan, and 

 was briefly described in the Annual Report of the Geological Survey 

 for that year 1 ; while at the same time appeared Mr. Teall's descrip- 

 tion, already referred to, of the Kentallen rock. Again, during last 



1 Ann. Eep. Geol. Surv. 1896 [1897] pp. 23 & 24. 



