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ME. J. A. THOMSON- ON THE HOENBLENDIC [NoV. I908, 



The contact-rocks on the eastern side of the boss are more regular 

 in appearance than those just described. The banding by the 

 garnets is quite parallel and continuous. The mineral composition 

 is, however, the same. In one rock an acid plagioclase was found 

 in addition ; it possesses highly-irregular boundaries, and is full of 

 inclusions. 



Pig. 9. — Biotite-muscoviie-garnet hornfels at the contact ivith ^oeridotite^ 

 Glendalougli, sJioiving biotite (in 2^^ocess of alteration into 

 chlorite), garnet, and magnetite, in a gi^oimd-mass of quartz and 

 muscovite. (Magnified SO diameters.) 



These rocks can best be described as hornfels. The contact- 

 phenomena round the amphibolite do not greatly differ from those 

 round the granite, except that the welding of the rocks has been 

 more complete in the amphibolite-aureole. Eound the granite 

 andalusite is very abundant, but this is due probably to original 

 differences of composition in the sediments. Tourmaline is also 

 found. 



y. The Hoenblendic Eocks of Geeystones. 



Prof. Watts described in 1893 a dyke of hornblende-peridotite 

 traversing the Ordovician rocks on the coast at Greystones. My 

 material was collected from two parallel dykes, a few hundred yards 

 south of the breakwater. Mr. Seymour informs me that he knows 

 of a third in the harbour. It seems probable that it was from this 

 that the sections in the Geological-Survey collection were made. 



