Correspondence— D. Mackintosh — A. Colvin. 95 



SCOTCH GRANITE ON WELSH HILLS. 

 SiR^ — Mr. D. C. Davies, F.G.S., is reported to have stated at a 

 meeting of the Geological Society on the 7th of January (after the 

 reading of my paper), that Scotch granite is to be found on the 

 Welsh mountains up to 2000 feet above the sea 1 Mr. Davies would 

 oblige, not only myself, but, I have no doubt, many of the readers 

 of the Geological Magazine, by mentioning the locality, and 

 whether the granite has been identified by tracing it northwards to 

 its source. D. Mackintosh. 



OEIGIN OF EARTHQUAKES. 



SiK, — I have only just been fortunate enough to see Mr. Malet's 

 interesting paper on the Origin of Earthquakes which appeared in 

 the February number of the Geological Magazine, 1873, Vol. X. 



p. 74:. 



I cannot think he has satisfactorily accounted for them, referring 

 them to the action of water percolating underground, wearing away 

 the rocks, and by so doing causing subterranean landslips. Surely 

 if it is to this that Earthquakes owe their origin, they would be 

 found occurring in every country, but far more frequently in those 

 in which is the greatest rainfall. Great Britain and Ireland are pro- 

 bably the most rainy countries in Europe, and accordingly ought to 

 be those most severely shaken by Earthquakes ; nevertheless, these 

 are countries nearly entirely exempt from such shocks. No doubt 

 many Earthquakes do occur in England every year, but of so slight 

 a character as to be perceptible only by special instruments for 

 detecting them, and Mr. Malet is evidently referring to those of a 

 more severe character. 



On the other hand, when we appeal to that observation of facts 

 on which Mr. Malet lays rightly so much stress, we find a close 

 connexion between the distribution of volcanic areas and the fre- 

 quency of earthquakes ; both volcanos and earthquakes have, in the 

 main, developed themselves in the same directions, and the latter 

 increase in frequency on approaching the focus of volcanic activity. 



Again, we hear of earthquakes ceasing when a new volcanic vent 

 has been formed in the district, or an old one re-opened. This is 

 apparently another indication of the community of origin of vol- 

 canos and earthquakes, the former acting as a safety-valve to the 

 latter, but by Mr. Malet's theory I can see no explanation of it. 



Again, we know that eruptions are attended and preceded by 

 earthquakes, though not of the first magnitude, but instead of con- 

 sidering the eruption the effect of the earthquake, it seems to me 

 more reasonable to assign them both to a common cause, namely, the 

 expansion of subterranean matter causing rendings in the rocks from 

 an over state of tension, and when, as in this instance, occurring at 

 a comparatively slight depth, the melted matter forcing a way to the 

 surface will flow over it as an eruption. The greatest earthquakes 

 frequently take place far from volcanic areas ; but we may, I think, 

 with some confidence refer them to the same cause producing the 

 minor shocks ; only that taking place at too great a depth for the 



