84 PROCEEDINGS OF THE BALTIMORE MEETING 



aftershocks, which were very numerous for a few days, but gradually became 

 less and less frequent. Shocks strong, but not nearly so strong as the first 

 one, occurred at 11.43 p. m. October 24 and at 5.44 p. m. November 12. After 

 that date they rapidly diminished in strength and frequency and soon ceased. 



The first shock was felt from the northern Lesser Antilles to the western 

 part of the island of Haiti — a distance of 300 to 350 miles. The first shock, 

 but none of the others, was followed by a sea-wave, which reached a height 

 of about 20 feet in the northwestern part of Porto Rico, and, its height in 

 general diminishing with the distance, was noticed for some distance along 

 the northern coast, over the whole of the western coast, and for a short dis- 

 tance on the southern coast. A number of people were drowned by this wave 

 and many small native huts were destroyed or displaced. 



The earthquakes are believed to be due to movements on a submarine fault. 

 The slope of the sea-bottom off the northwestern part of the island is so steep 

 that one is almost driven to assume that a great fault-scarp exists there. The 

 movement at the time of the first shock must have had a vertical component 

 to generate the strong sea-wave. 



Although both the Greater and Lesser Antilles are subject to strong earth- 

 quakes, the island of Porto Rico seems to have experienced only one other 

 severe disturbance since it was discovered by Columbus ; this disturbance was 

 also submarine to the east of Porto Rico and immediately south of the island 

 of Saint Thomas. The shock inaugurating this disturbance occurred on No- 

 vember 12, 1867. It was rather more severe than the recent shock, and was 

 also followed by a more important and more extensive sea-wave. 



(A full report will be made to the Insular Government, and technical details 

 will be published in the Bulletin of the Seismological Society of America.) 



Presented by the senior author extemporaneously. 

 Discussed by Prof. C. P. Berkey. 



STRUCTURE OF THE PACIFIC RANGES, CALIFORNIA 

 BY BAILEY WILLIS 



iAl)Stract) 



The term Pacific ranges of California is here used to designate the Coast 

 ranges and the Sierra Nevada. Their structure is described as an effect of 

 compressive stress, but is contrasted with the structure of the Appalachians, 

 whereas in the latter the effects of compression are folds and low angle over- 

 thrusts; in the Pacific ranges the dominant structure is the rotated mountain 

 block guided by a high angle upthrust. 



Ben Lomond Ridge, in the Santa Cruz quadrangle, the Santa Inez Range, 

 west of Santa Barbara, and the Sierra Nevada are cited as examples of ro- 

 tated mountain blocks. In each case their visible surfaces are two — an old 

 topographic surface, which has been tilted, and a fault-scarp, which has been 

 more or less eroded. Rotation of the block is regarded as demonstrated by 

 the growth of consequent streams and canyons on the old topographic surface, 

 and also by normal faults, similar to landslides, in the direction in which tilt- 

 ing would cause downslipping. 



Rotation has resulted in the uplift of one edge of the block — the edge in 



