456 PHILOSOPHICAL TRANSACTIONS. [aNNO I76O. 



an instance to the same purpose, where the direction of the ridge varies about a 

 point from due north and south, lying nearly from n. by e. to s. by w. There 

 are many more instances of this to be met with in the world, if we may judge 

 from circumstances, which make it highly probable that it obtains in a. great 

 number of places, and in several they seem to put it almost out of doubt. The 

 reader is not to suppose however that in any instances the highest rise of the 

 ridge, and the inclination of the strata from thence to the countries on each 

 side, is perfectly uniform; for they have frequently very considerable inequalities, 

 and these inequalities are sometimes so great that the strata are bent for some 

 small distance, even the contrary way from their general inclination. 



At considerable distances from large ridges of mountains, the strata for the 

 most part assume a situation nearly level ; and as the mountainous countries are 

 generally formed out of the lower strata, so the more level countries are gene- 

 rally formed out of the upper strata of the earth. Hence in countries of this 

 kind the same strata are found to extend themselves a great way, as well in 

 breadth as in length : we have an instance of this in the chalky and flinty coun- 

 tries of England and France, which (excepting the interruption of the channel, 

 and the clays, sands, &c. of a few countries) compose a tract of about 300 miles 

 each way. 



Besides the raising of the strata in a ridge, there is another very remarkable 

 appearance in the structure of the earth, though a very common one ; and this 

 is what is usually called by miners the trapping down of the strata; that is, the 

 whole set of strata on one side a cleft are sunk down below the level of the cor- 

 responding strata on the other side. If in some cases this difference in the level 

 of the strata, on the different sides of the cleft, should be very considerable, it 

 may have a great effect in producing some of the singularities of particular earth- 

 quakes. 



Part II. In the former part of this essay, having recounted some of the 

 principal appearances of earthquakes, as well as those particulars in the structure 

 of the earth, on which he supposed these appearances to depend, Mr. M. 

 thinks it is sufficiently manifest that in some instances, at least, earthquakes are 

 actually produced by subterraneous fires ; it therefore remains to be shown how 

 all the appearances above recited, as well as many other minuter circumstances 

 attending earthquakes, may be accounted for from the same cause. 



Sect. 1. The returns of earthquakes in the same places, either at small or 

 large intervals of time, are consistent with the cause assigned: subterraneous 

 fires, from their analogy to volcanos, might reasonably be supposed to subsist for 

 many ages, though we had not those instances already mentioned which put the 

 matter out of doubt. And as it frequently happens that volcanos rage for a time, 

 and then are quiet again for a number of years; so we see earthquakes also fre- 



