1888.] 
Dr Stecher on Contact- Phenomena. 
163 
out, underwent re-solution in the magma ; the latter being rendered 
acid by the assimilation of acid material. If this material were 
added in sufficient quantity, and the magma were maintained during 
an enormous period of time at a uniform temperature, all the 
olivine would be dissolved; while in a rapidly consolidating mass 
such a destruction of the olivine is impossible. 
In all probability, there is a close analogy between these facts and 
the data given by Liebe and Zimmermann,* Rohrbach,f &c., who 
regard the olivine as a so-called endogenous contact-product. Such 
an interpretation, unsatisfactorily established, is only apt, I think, 
to lead us to another problem. It seems to me more advisable to 
apply the above explanation to those olivines which have been 
proved to be endogenous contact-products. We may thus obtain 
further support for the opinion just expressed. 
The supposition that allothigenic material has been corroded and 
eaten into by the igneous magma is supported by the following 
observations : — The Salband of the olivine-diabases of Hound Point 
and Salisbury Crags envelops numerous fragments of the associated 
rocks. The fragments vary in size from minute microscopic grains 
up to great masses, some of which have been figured by Professor 
Geikie in the sections he gives {loc. cit.). At a short distance 
from the contact the diabase invades the enclosed quartz grains in 
a sinuous manner, and there occur aggregations of quartz grains 
which have the appearance of being mechanically welded together. 
Professor Geikie { has remarked that large fragments of sedimentary 
rocks must have been melted down by the igneous magma. But 
the slow-cooling in the central portion of the volcanic masses 
favoured the neutralisation of the acid substance which had 
been added to the original basic mass; nearer the contact such 
chemical changes could not occur. In this latter case, a surplus of 
silica remained, which separated and crystallised as automorphic 
quartz (Hound Point, 12 feet above base). The specimens from 
these parts of the massive rock cease to exhibit anything that 
might suggest the earlier existence of olivine. The volcanic 
* Liebe and Zimmermann, “ Die jiingeren Eruptivegebilde im S. W. 
Ostthiiringens,” Jahrh. d. preuss. Landeranstalt, 1885, p. 178. 
t Rohrbach, “ Ueber die Eruptivgesteine im Gebiete d. ma’hr. Kreideform,” 
Tschermak' s Mineral, u. petr. Mitth., 1885, vii. pp. 27 and 54. 
X Text-Book, pp. 535, 536. 
