DISCUSSION AND CONCLUSIONS. 69 



Rhaetikon, which he held to be clearly associated with hnes of thrust (a conclusion challenged 

 by Von Seidlitz '06). Paulcke ('04) so explains the green-rocks of the Antirhaetikon, and 

 Becke ('03) the peridotites of the Tauern, which invade calc-phyllites and green schists which 

 are in part Mesozoic. Unfortunately the writer has not seen Staub's ('15) discussion of this 

 problem to which reference should be made. Similar rocks in California are held to be intrusive. 



Steinmann ('05) basing his view chiefly on the features of the Engadine, is opposed to 

 this conclusion. In general, he urges, there is no association between the occurrence o' the 

 green-rocks and the great recumbent sheet-folds, but they are confined to the Rhaetic sheet 

 only, and were carried forward passively when this was folded up. The root-region belongs 

 to the inner zone of the Alps and was probably the Ivrea zone. In the Rhaetic sheet there 

 occur radiolarian cherts and clays, which he assumes are of abyssal origin, and these are overlain 

 by shallow-water Cretaceous sediments. There must therefore have occurred an uplift of 

 the sea-floor to the extent of ten or fifteen thousand feet in early Cretaceous times. The 

 association of Jurassic radiolarite and shallow-water Cretaceous deposits occurs in no other 

 region of the Alps, and we may therefore consider that the extrusion of the ophiolitic rocks 

 accompanied or followed closely after this great uplift, and that they are for this reason 

 confined to the Rhaetic sheet. It was in the later intense overfolding of Tertiary times 

 that they were carried forward into their present position and were strongly metamorphosed. 

 "Thus the circumstances of their intrusion were different from those of granite and diorite 

 massifs and dike rocks, and we may conceive that under the great sea depths basic magmas 

 collect, and are injected during the folding of these abyssal regions, while more acid magmas 

 rise into the foundations of the continents and the regions of shallow seas" (Steinmann '05, 

 p. 59). This hypothesis, therefore, depends on the necessarily abyssal character of the radio- 

 larites of the Alps, and this assumption is also made by Suess, though he calls attention to 

 an area in the eastern Alps (Suess IV, p. 190) (also discussed by Uhlig '11), where they are 

 very intimately associated with littoral sediments. There is an increasing body of evidence 

 put forward by Lawson ('95), Walther ('97), David ('99), Martin ('07), Dixon ('11), and most re- 

 cently by Davis ('18), which makes it clear that radiolarian cherts are frequently formed under 

 shallow-water conditions, this being notably the case in California, which was considered by 

 Steinmann to illustrate his hypothesis. 



Suess ('04, IV, pp. 248, 564) objects to Steinmann's hypothesis because of other evidence 

 of the development of green-rocks in folded mountains, where deep-sea conditions are inadmis- 

 sible, namely the occurrence of green-rocks as sills in the shallow-water Mesozoic rocks with 

 the gypsiferous and saline Triassic beds of the Pyrenees, Morocco, and Tunis, and holds instead 

 that they "form sills in dislocated mountains, which sometimes follow the bedding planes 

 and sometimes the planes of movement," believing that the intrusion actually accompanied 

 the dislocation. (Suess IV, p. 561-567). The fact that they are confined to the Rhaetic 

 sheet in the Alps is explained by reference to the nature of its component rocks. In the case 

 for example, of the Simplon area, as soon as they had reached the upper limit of the gneiss they 

 discovered the planes of least resistance in the Triassic limestones and the Jurassic calc-phyllite, 

 generally the bedding-planes, and spread out in them. For this reason they are intercalated 

 in the Mesozoic sediments one above the other. Ascending dikes are rare (ibid., pp. 134). So 

 also they were injected into weak structures in northern Africa, and were brecciated by later 

 movements (ibid., p. 222). Probably the alpine green-rocks were rooted in the Ivrea zone 

 of intrusions, which were developed during movement on the sole-plane, along which the Dina- 

 rides were thrust on to the Alps, but it is possible that this question can never be settled by 

 observation, for the connection with the south has now been destroyed (ibid., pp. 154, 564). 



It is very clear from many instances we have described how correctly are the basic and 

 ultrabasic rocks described as concordant sills in dislocated mountains, developed during the 

 crust-movements, and so far Suess's conclusion is confirmed, but its application appears to 

 be carried too far when it is sought to include all the alpine green-rocks. In particular, it 

 does not seem permissible to regard the finer grained green-rocks, such as those of the Simplon 



