Site of the Ancient City of the Aurunci, 239 



greater pressure to which it was subjected during the act of 

 cooling. 



Abich, however, has found it necessary to distinguish a class 

 of formations intermediate between trachytes and green stones, 

 which he denominates trachyte-dolerite. To this he refers 

 the rocks which encircle the peak of Teneriffe, those of one 

 of the volcanoes in Kamschatka, of the little cluster of islands 

 between Lipari and Stromboli described by Hoffmann, and 

 above all the material which constitutes the Monte della 

 Croce, the central cone of Rocca Monfina already alluded to. 

 Abich considers the felspar present in this rock to be oligo- 

 klase, which, by reference to the table, will be found to be a 

 bisilicate, and the many green specks of augite which per- 

 vade it indicate a further change in the composition of the 

 mass, and a nearer approach to greenstone. With this latter 

 material, which, as we have seen, is a compound of augite 

 with one of the species of felspar poorest in silica, the rock 

 called basalt must not be confounded ; as in it we may re- 

 cognise a still further step in the elaboration of the consti- 

 tuents, this substance being composed of an intimate mixture 

 of augite and magnetic iron with a mineral of the zeolitic 

 family. The composition of the latter is such as to imply 

 that it may have been formed out of labradorite with the ad- 

 dition of water, the presence of which in all zeolites is the 

 cause of that bubbling up under the blowpipe which has oc- 

 casioned them to be distinguished by that general appellation. 



"We perceive a similar change in the rock called clinkstone, 

 which has been shewn by Gmelin to be an intimate mixture 

 of glassy felspar with a zeolite. 



Thus, as we proceed towards the more modern groups of 

 volcanic formations, we find new ingredients successively 

 coming into play ; first, the alkalies increasing, then lime 

 and magnesia becoming part of the constitution of the mineral 

 mass, and, lastly, water entering into combination with the 

 earthy materials. 



The gradual increase of soda is likewise a remarkable cir- 

 cumstance, modern lavas appearing to contain a much larger 

 quantity than the volcanic products of ancient periods, and 



