23 



that are too easily adopted on the local causes 

 of earthquakes. 



In all those places where the soil has been in- 

 cessantly agitated for whole months, as at Ja- 

 maica in 1693*, Lisbon in 1755, Cumana in 1766, 

 and Piedmont in 1808, a volcano is expected to 

 open. People forget, that it is far from the sur- 

 face of the Earth we must seek the focus or 

 centre of action ; that, according to undeniable 

 evidence, the undulations are propagated almost 

 at the same instant across seas of an immense 

 depth, at a distance of a thousand leagues ; and 

 that the greatest commotions take place not at 

 the foot of active volcanoes, but in chains of 

 mountains composed of the most heterogeneous 

 rocks. We have given in the preceding book a 

 geognostical description of the country round 

 Caraccas ; we there find gneiss, and mica-slates, 

 containing beds of primitive limestone. The 

 strata are scarcely more fractured or irregularly 

 inclined than near Freyberg in Saxony, or wher- 

 ever mountains of primitive formation rise ab- 

 ruptly to great heights. I there found neither 

 basaltes nor dolerite, nor even trachytes or trap- 

 porphyries ; nor in general any trace of an ex- 

 tinguished volcano, unless we choose to consider 

 the diabases or primitive gruenstein, contained in 

 gneiss, as masses of lava, which have filled up 



* Phil. Trans, for 1694, p. 09. 



