372 Messrs. Milne and Gray's Experiments 



entrance of the bay, in the extreme south, the foldings are 

 more steep, and the faults exceedingly common. At many 

 points in the faulted districts distinct evidence of a very 

 recent elevation are to be seen in the form of borings of 

 Lithodomi 6 to 10 feet above high-water mark. In some of 

 these holes the shells still exist. As the rock is exceedingly 

 soft, and at the same time exposed to the weather, it is reason- 

 able to suppose that the elevation must be of a very recent 

 date. 



It is in this district of faults and marks of recent elevation 

 that we have determined an origin for several of the more 

 recent earthquakes. 



Classification of Earthquakes* 



Instead of classifying earthquakes according to the im- 

 pression they have on our senses, we would suggest that they 

 might be better classified by the records we obtain from our 

 seismographs. So far as records have gone, it would appear 

 that we might classify shocks into (1) Direct, (2) Transverse, 

 (o) Compound shocks. 



By a direct shock we mean a shock consisting of vibrations 

 in the direction of propagation, the degree of horizontality 

 being expressed by the ratio of the horizontal to the vertical 

 motion. 



By a transverse shock we mean a shock which is for the most 

 part due to vibrations at right angles to the direction of pro- 

 pagation. The character of this might be expressed in the 

 same manner as the character of the direct shock. 



By a compound shock we mean a shock showing combina- 

 tions of the direct and transverse vibrations. The horizontality 

 might be expressed as before. 



In defining an earthquake we should consider it as the 

 result of an irregular disturbance in the earth's crust, generally 

 producing both waves of compression and distortion. In 

 certain cases the former of these may be prominent, whilst in 

 other cases the latter. 



Frequency of Earthquakes at different Seasons. 



The following Table, extracted from the records of the 

 Meteorological Observatory, is a good summary of the number 

 and intensity of earthquakes felt in Tokio, as recorded by one 

 of Palmieri's instruments during the last four years. 



