ORIGIN OF CERTAIN CONTACT ROCKS. 185 



The variability of the contact rocks at any point will, upon 

 this hypothesis, depend upon three main factors : — 

 (i.) The character of the acidic emanations, 

 (ii.) The variations of temperature and pressure along 



the conduit, 

 (iii.) The composition of the basic rocks traversed. 



Of these factors, the first is most readily disposed of with 

 regard to one group. It is probably the most potent influence 

 in determining the type of the resultant contact rock. The 

 presence of boron in the emanations is undoubtedly necessary for 

 the creation of the limuritic type. What other mineralisers are 

 predominant in the creation of the other tj^es is not so clear. 



The influence of the second factor cannot yet be said to have 

 been determined in any detail. 



The effect of the third factor can be investigated only by 

 means of detailed analytical work, which has not yet been begun. 



In connection with the hypothesis here advanced, it may be 

 of interest to observe that a certain amount of collateral evidence 

 may be cited. 



If, as the author beheves, the peculiar contact rocks here 

 mentioned are formed by the assimilation of material from the 

 basic rocks by the magmatic emanations issuing from the acidic 

 magma hearths, it is logical to enquire whether the basic rocks 

 themselves exhibit throughout their mass signs of the operation 

 of these emanations, and whether other veins (apart from those 

 with which this paper deals) in the neighbourhood of the basic 

 rocks exhibit mineralogical characteristics which may indicate 

 the derivation of some part of their contents from these rocks. 



The basic rocks which attained their mise en place at a date 

 slightly earlier than that of the acidic types are of extremely 

 variable character in all the observed localities in which they have 

 been observed. Perhaps the most widely distributed rock type is 

 serpentine, though gabbros, norites, and pyroxenites, together with 

 varieties into the composition of which amphibole largely enters are 

 also common. The serpentine rock exhibits some differences of 

 character at different points ; and although often almost entirely 

 composed of serpentine itself it shows in places the presence of 

 carbonated minerals and chlorite in addition. In the North Dundas 

 district calcite, dolomite, and chlorite have been recorded ^ At 

 Dundas a new mineral — stichtite — allied to pjo-oaurite^ is found in 

 the serpentine in veins, blebs and irregularly-shaped masses. The 

 serpentine both at Anderson's Creek and North Dundas is traversed 

 by numerous veins of asbestos and magnetite, in which these 

 minerals are arranged in threads or columns at right angles to the 

 fissures, which they fill after the manner of crustified veins. The 

 presence of the acidic rocks in the immediate vicinity of these 

 occurrences mav be of rausal signifi^-anrp.s j^ jc, ^\ lea^t possible 



1 Vide : Geol. Surv. Tas., Bulletin No. 6, p. 23. 



2This mineral was formerly supposed to be Kammererite. Viite : \V. F. Petterci, " Catalogue 

 of the Minerals of Tasmania," 1910, pp. 167-170. 



3 Compare the observations of J. A. Dresser in Eastern Quebec : " Economic Geology," Vol. 

 IV., 1909, pp. 130-140. 



