24 THE OLD RED SANDSTONE. 
granite, the terminating zone of porphyry. The elevating 
power appears to have acted in the centre, as in the well- 
known case of Jorullo, in the neighborhood of the city of 
Mexico, where a level tract four square miles in extent rose, 
about the middle of the last century, into a high dome of 
more than double the height of Arthur’s Seat.* In the forma- 
tion of our Scottish mountain, the gneiss and mica-slate of the 
district seem to have been upheaved, during the first period 
* It is rarely that the geologist catches a hill in the act of forming, 
and hence the interest of this well-attested instance. From the period 
of the discovery of America to the middle of the last century, the 
plains of Jorullo had undergone no change of surface, and the seat of 
the present hill was covered by plantations of indigo and sugar-cane, 
when, in June, 1759, hollow sounds were heard, and a succession 
of earthquakes continued for sixty days, to the great consternation 
of the inhabitants. After the cessation of these, and in a period 
of tranquillity, on the 28th and 29th of September, a horrible subter- 
ranean noise was again heard, and a tract four square miles in extent 
rose up in the shape of a dome or bladder, to the height of sixteen 
hundred and seventy feet above the original level of the plain. The 
affrighted Indians fled to the mountains ; and from thence looking 
down on the phenomenon, saw flames issuing from the earth for miles 
around the newly-elevated hill, and the softened surface rising and 
falling like that of an agitated sea, and opening into numerous rents 
and fissures. Two brooks which had watered the plantations precip- 
itated themselves into the burning chasms. The scene of this singular 
event was visited by Humboldt about the beginning of the present 
century. At that period, the volcanic agencies had become compara- 
tively quiescent; the hill, however, retained its original altitude; a 
number of smaller hills had sprung up around it; and the traveller 
found the waters of the engulfed rivulets escaping at a high tempera- 
ture from caverns charged with sulphureous vapors and carbonic acid 
gas. There were inhabitants of the country living at the time who 
were more than twenty years older than the hill of Jorullo, and who 
had witnessed its rise. 
