Maclaren. — On Castle Bock, Coromandel. 



215 



perhaps predominating. Length, up to 2 mm. Carlsbad 

 twins are common among the orthoclase feldspars. Plagio- 

 clase feldspars twinned polysyuthetically, but not numerously. 

 Amphiboles, light-green and porphyritic, reaching to 10 mm. 

 in length. Many show strong resorption borders. Magnetite 

 common. Free quartz sparingly present. The following is 

 the determination by chemical analysis : — 



99-75 



From the above characteristics, I have named the rock a 

 hornblende trachyte, but it must be confessed that it stands 

 petrologically on the border-line between trachytes and ande- 

 sites. 



Before concluding these notes I take the opportunity of 

 drawing attention to a point that, so far as I am aware, has 

 escaped the observation of geologists who have dealt with the 

 igneous rocks of the Hauraki Peninsula. Though the Thames 

 andesites have often been compared with those of Transyl- 

 vania, and of the Com stock region of North America, yet the 

 closer parallel drawn between the rocks of the two above- 

 mentioned localities, and also of North Germany, has not yet 

 been applied to the succession of igneous rocks on the Hauraki 

 Peninsula. So struck was Von Richthofen by the almost 

 invariable order of succession of volcanic rocks that he 

 enunciated the following as a natural law : The first-ejected 

 lavas in a district are the andesites. Succeeding these are 

 trachytes ; and in the final stages of eruptive action the acid 

 rhyolite lavas and the basalts are poured forth. Now, 

 applying this law (to Vv'hich, indeed, there are many excep- 

 tions) to the rocks of the Hauraki Peninsula, we find that 

 the first deposits were the auriferous andesites, mainly de- 

 veloped on the western slopes of the range. In Miocene 

 times we have the trachytic breccias of Beeson's Island and 

 of Port Charles, &c., typically developed in the localities 

 named, and we have also in this period the trachyte dykes 

 of Castle Rock and Torehine, and finally, to complete the 

 parallel, we have in the rhyolites of Tairua and the south- 

 eastern portion of the peninsula the youngest of the volcanic 

 rocks occurring in the area under discussion. 



