20 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. 1Meuoie ^l ti xix; 



this work has been given by Brauns ('09). While in some respects the mode of occurrence and 

 range of petrographical characters exhibited is analogous with those of the rocks of similar age 

 in Devonshire, as pointed out by Flett and Dewey ('11), there is but rarely much albitisation 

 and generally no association with radiolarian chert. The intrusions in the Middle Devonian 

 rocks comprise essexite (in one instance containing 5.48 per cent of Na 2 0, 3.77 per cent of K 2 0, 

 and 48.79 per cent of SiO,) , essexitic diabase, mica-diabase, hornblende-diabase, and amphibole- 

 picrite (generally in independent intrusions, but in one instance occurring within diabase, into 

 which it passes by gradual transition). With these partly alkaline intrusions are associated 

 porphyritic diabases, not unusually sodic, but containing epidote and secondary albite, though 

 the primary feldspar is labradorite or bytownite (Heineck '03) ; flows of dense and vesicular 

 diabase-schalsteins, and tuffs with volcanic bombs. The keratophyres ("lahnporphyries") 

 vary betwen highly sodic types (one of which Prior described as a reibeckite-tinguaite) and 

 quartz-keratophyres, hi which potash predominates. The great variation even in a single 

 mass between the proportions of the two alkalies is very characteristic of the series, and may be 

 due to a varying degree of secondary albitization, as Flett and Neithammer ('09) would suggest. 

 The presence of reibeckite and aegyrine was urged by Erdmannsdoerfer ('07) as strong evidence 

 of their alkaline character. The Upper Devonian intrusions are much less varied. The diabasic 

 rocks frequently show pillow-structure, and hornblende is absent from both the diabases and the 

 associated picrites. No sign of alkaline minerals occurs. (Brauns '04, '06 ; Reuning '07.) 



In the Harz Mountains no distinction has yet been observed between an earlier alkaline 

 and later calcic series of Devonian igneous rocks, though both sodic and normal types occur. 

 Erdmannsdoerfer ('09) has shown the consanguinity between the diabase of this region and the 

 distinctly alkaline keratophyre, which contains both potassic and sodic types. In the latter 

 albite, soda-orthoclase, aegyrine, and arfvedsonite are present. The Devonian igneous rocks 

 of the Fichtelgebirge present an analogous range of types; according to the most recent studies 

 of Riman ('07) and Weber ('10) diabase, leucophyres, spilites, and keratophyres are present, 

 together with numerous sills and some dikes of picrite, which have been elaborately studied by 

 Uhlemann ('09). 



The Carboniferous period saw the great Variscan folding following on this long period of 

 geosynclinal depression, and considerable crushing and even great overthrusting occurred 

 (Kayser '00). This rolled out the igneous rocks, both massive and pyroclastic, into schalsteins, 

 which occur on the Rhine (Milch '89), in the Harz, and in the Fichtelgebirge (Lossen '82, 

 Pelikan '98, '99) . Accompanying this folding were great plutonic intrusions in the Harz and 

 in Saxony. The massif of the Brocken is a differentiated complex (Lossen '82 Erdmannsdoerfer 

 '05), consisting of peridotite, gabbros, and norites followed by less basic and finally acid granites. 

 The basic and ultrabasic masses are disposed in roughly stratiform masses approximately 

 parallel to the direction of Variscan folding. The granites, the intrusion of which occurred 

 before the gabbros had cooled, have much more transgressive boundaries, and the latest dikes 

 appear to follow the Hercynian structure-lines and run almost perpendicular to the Variscan 

 direction. (See fig. 6.) Veins of nephrite fill fissures in the harzbergite which also run parallel 

 to the Hercynian direction, and therefore transversely to the longer axes of these ultrabasic 

 rocks (Uhlig '10). The norite is rich in biotite, and in it occurs the biotite-peridotite found by 

 Koch ('89) in the Kaltenthal, which Rosenbusch ('07) classsed doubtfully with the alkaline 

 rocks. A brief examination of the locality suggested to the writer that it may be merely a 

 schlieren band in an olivine-biotite-norite. It has no association with any alkaline features. 



In the Odenwald, there is also a differentiated series of intrusions into liighly 

 folded Devonian rocks. They vary from ultrabasic to acid types, with a stratiform arrangement 

 most marked in the basic members, e. g., at Beerbach. Though there have been claimed 

 to be passage-rocks between the basic and acid members, those rocks of intermediate position 

 and composition, seen by the writer near Darmstadt, appeared rather to be hybrids; they have 

 a markedly blotched appearance and resemble the hybrid rocks in Skye, and the rocks 

 believed by Dr. Harker to be such in the Glen Doll complex and in Mull. The "picrite" of 



