176 PROCEEDINGS OF SECTION C. 



16— THE ORIGIN OF CERTAIN CONTACT ROCKS WITH A HIGH 

 CONTENT OF LIME AND MAGNESIA. 



By L. KEITH WARD, B.A., B.E. 



CONTENTS. 



I. — Introduction. 

 II. — Classification and Description of the Rocks — 



A. Diopside and Chlorite Rocks. 



B. Axinite-Actinolite-Calcite and Vesuvianite-Garnet- 



Actinolite Rocks. 

 III.— The Mode of Occurrence of the Contact Rocks. 

 IV. — Hypothesis with Regard to Origin. 

 V. — Conclusion. 



I. — Introduction. 



In this paper the author proposes to deal briefly with certain rocks 

 which in his belief owe their special features to the contact meta- 

 morphic effects of the Devonian granite of Tasmania. 



No comprehensive study having yet been made of all the known 

 developments of the Devonian granite and its border zones, the 

 conclusions hereafter cited must necessarily rest upon the evidence 

 acquired in localities in which investigations have already been 

 made. Moreover, in the absence of quantitative chemical analyses 

 of the granite the contact rocks themselves, and the rocks invaded 

 by the granitic magma, the discussion of the question of the origin 

 of the contact rocks can be based only upon the quantitative 

 characters of these rocks and their actual mode of occurrence. 



Nevertheless, the author beheves that sufficient evidence has 

 been acquired to justify the formulation of a very definite hypo- 

 thesis regarding the genesis of certain peculiar mineral groupings, 

 and to suggest the possibility of the formation of certain specific 

 mineral aggregates from entirely different sources. 



With regard to the essential processes involved the views of the 

 author are in entire sympathy with the expressed opinions of Prof. 

 A. Lacroix with respect to the contact phenomena exhibited by the 

 granite of the Pyrenees^. 



Certain rock types mentioned are very similar to those described 

 by Prof. A. Lacroix, though the source of the greater part of the 

 lime and magnesia content of these rocks is, according to the 

 hypothesis here advanced, distinct from that claimed by Prof. 

 Lacroix for the similar rocks in the Pyrenees. 



II. — Classification and Description of the Rocks. 



A number of rather uncommon mineral aggregates are here 

 considered together. The aggregates, as will be seen from the 

 following condensed descriptions, are complex. Yet the occur- 

 rences of the complex aggregates are not isolated, for recurrences 

 of essentially identical types have already been observed in different 

 districts or different parts of the same district. 



1 Vide Bulletins des Services de la Carle Geologique de la France, Nos. 64, 71. 

 Also, Sir A. Geikie : "Textbook of Geologv," IV. Kditioii, Vol. H, p. 780; and J. Geikie, 

 " Structura 1 and Field Geology," pp. 215 and 217. 



