368 Transactions. — Geology. 



hard black andesite. These have consolidated at an earlier 

 period, and have been caught up by the liquid lava as it 

 ascended the pipe of the volcano. These blocks look as if 

 they had been rounded by water, and have almost the appear- 

 ance of concretions. They are in most cases surrounded by 

 concentric layers of decomposed rock. Probably this is due to 

 the interaction of the fluid lava on the inclusions, and this has 

 produced a rock which has weathered in concentric layers, 

 the junction of the two rocks affording a weak place where 

 atmospheric water might readily promote decomposition. 



The rock is of a dark-grey colour, but very often shows a 

 tint of red, which disappears on long exposure to the air, as 

 can be seen m the stone in the buildings above mentioned, 

 where the newer parts can easily be distinguished from the 

 older by the peculiar tint of the stone. The rock is full of 

 cavities and steam-holes', but none of these were noticed to be 

 filled with infiltration products. This rendered an_ accurate 

 determination of the specific gravity almost impossible. On 

 weighing pieces in distilled water the following results were 

 obtained : — 



I., 2-621. II., 2-619. Average, 2-620. 



On grinding to a fine powder, and using a specific-gravity 

 bottle, a number of different weighings gave results between 



2-67 and 2-68. 



The rock appears to the eye to be composed of a glassy or 

 finely crystalline ground-mass in which crystals of felspax are 

 porphyritically distributed. Some of these are over half an 

 inch long, but as they crumble readily it is impossible to detach 

 more than mere fragments for examination. They show at 

 times a fine striation and glistening surfaces, but the cleavage 

 is not well marked. 



The examination of the rock was conducted in two 



ways : — 



A. A chemical analysis of the rock as a whole, and of the 



felspar separately. 



B. A microscopic examination by means of thin sections. 



A. Chemical Analysis. 

 •- Portions of the rock were taken and ground up, so that a 

 perfectly general sample might be obtained, and were analysed 

 a number of times. Since the rock was roughly examined 

 first under the microscope a qualitative chemical analysis was 

 unnecessary. I show all the results obtained, with the excep- 

 tion of those in No. III., as they were completely wrong, from 

 known reasons. The last analysis is the most trustworthy, 

 for reasons which I shall point out : — 



