﻿602 DE. A. p. YOUNG OX THE STEATIGEAPHY AND [NoV. I908, 



are some which show the ophicalcitic structure, and thus bear 

 witness to the ingress and complete consolidation of a portion of 

 the serpentine-magma at a date prior to a final stage of the eruption 

 in which the fragments were detached and carried off. 



Last Stage of the Intrusion. 



The final accession of fluid magmas is represented by a core of 

 serpentine which has cr^'stallized by slow cooling in a state of rest, 

 unsolicited by forces of shearing or stress. This is proved by the 

 abundant remains of large pyroxene-crystals showing no direction 

 in their arrangement, and by numerous pseudomorphs in bastite 

 after pyroxene in composite crystalline growths which evidently 

 still retain their original forms. 



In the younger parts of the rock an occasional slight strain in 

 the pyroxenes and some instances of cataclastic structure are the 

 only indication of the survival of forces, which were at the last too 

 attenuated to impose schistose structure on the rock. 



The rock must have consolidated nearly in its present form 

 while still in communication with the main magmatic reservoir, and 

 before the commencement of the journey from the original to the 

 present site. 



The original igneous rock has been affected chemically, notably 

 by serpentinization on an extensive scale. Some of the changes 

 may possibly have taken effect during translation, but the change 

 of form has been unimportant. The ophicalcite obviously acquired 

 its banded structure on the original site, and has been translated 

 without perceptible deformation. The same is true of the masses 

 of indurated contact-schist. 



It appears to me to be evident that the whole mass of serpentine, 

 ophicalcite, and indurated schist still hangs togerher with the 

 original relative positions of its parts, and that in the course of 

 translation it has undergone no deformation and no interruption 

 of continuity, beyond that due to minor faults and fractures. 



SUMMAET OF Co^"CL"D SIGNS. 



As respects the general structure of the Tarnthal mass, my 

 reading of it is as follows : — 



The rock -series of the mass may be divided into three parts : — 



(1) A lower section consisting of (1 a) principal dolomite 

 (Hhsetic) and (1 b) Liassic limestone, the upper beds being the 

 youngest. This lowest portion is in normal position, and is 

 scarcely disturbed. 



(2) A middle section consisting of (2 a) calcareous schists, (2 b) a 

 band of massive dolomite and dolomitic breccia, and (2 c) calcareous 

 schists with green bands. This section shows marks of violent 

 distortion and crushing. 



