9 
GENERAL REPORT ON INVESTIGATIONS IN PORTO RICO. 
GEOLOGY. 
Mr. Robert T. Hill gives the following account of the island: 
Porto Rico consists of three geologic elements: (1) A central system of deeply ribbed and corru- 
gated mountains, with V-shaped gorges and ridges; (2) lower hills, forming irregular bands along the 
north and south coast; (3) playa plains, consisting of alluvial soil, occupying old reclaimed estuaries, 
which extend from the foot of the central mountains across the line of foothills to the seashore. It 
is roughly estimated that nine-tenths of the island is of the mountainous character, and that the 
remaining tenth is of the foothill and playa character. The central mountains are composed of water- 
sorted volcanic ejecta — tufas and conglomerates — with occasional dikes and masses of interbedded 
subcrystalline bluish limestone of rare or exceptional occurrence, all of which is entirely decayed at the 
surface, breaking down into red clay resembling that of the southern Appalachians. The dikes are of 
hard, black, igneous rock, with small white porphyritic. crystals. In the east the substructure is said 
to be granites, especially syenites. In Naguabo and in Mayaguez some serpentine rocks have been 
observed. The foothills are composed exclusively of rocks of sea origin, consisting of the peculiar type 
of tropical white limestones, of a loose-textured, chalky, marly, and shelly nature, of various degrees of 
induration. The rocks of the central mountain region are of Cretaceous and possibly early Eocene age; 
at least, no evidence tending to establish other dates for their formation has been as yet discovered. 
The white limestones of the coastal hills are all of later Tertiary and Pleistocene age. The playa 
deposits are alluvial formations, consisting usually of a rich, chocolate-colored, sandy loam. 
CLIMATE. 
Porto Rico is well within the torrid zone, but, lying toward the eastern limits of 
the Greater Antilles, its climate is favorably modified by the prevailing winds and 
rendered far more agreeable than in any other island of the W est Indies. The trade 
winds, modified by the high mountains of the interior of the island, give it not only a 
moderate temperature, but they cause the peculiar distribution of precipitation which 
characterizes it. Though the area of the island is small, nevertheless the extremes of 
climate, particularly the rainfall, are very great. The annual precipitation on the 
southern coast seems to be only about 20 inches, while on the northern side it ranges 
from 55 to 123 inches. It is even said that there is an authentic record for 13 months, 
chiefly in 1893, when notan inch of rain fell at Guayama, and the records at Cabo Rojo 
show one period of three years when no rain fell. This difference in precipitation 
between the north and south sides of the island is shown by the character of the beds 
of the streams, those of the south being more dry and full of rocks and bowlders. 
In the following tables are given certain meteorological data for seven places in 
Porto Rico, compiled from the reports of the United States Weather Bureau. 
Climatological data for year ending April 30, 1900. 
Temperature. 
Precipitation. 
Sky. 
Locality and 
month. 
Mean. 
High- 
est. 
Date 
of 
high- 
est. 
Low- 
est. 
Date 
of 
low- 
est. 
Total. 
Greatest 
in 24 
hours. 
No. of 
rainy 
days. 
No. of 
clear 
days. 
No. of 
partly 
clear 
days. 
No. of 
cloudy 
days.' 
Prevailing 
direction 
of wind. 
San Juan : 
O/P 
°F. 
°K 
Inches. 
Inches. 
May 
78. 7 
89 
3 
68 
1 
2. 59 
0.85 
12 
11 
18 
2 
SE. 
.1 one 
79.4 
91 
22 
71 
6 
7.23 
1.42 
28 
7 
17 
6 
E. 
•Inly 
79. 7 
87 
2 
70 
8 
7.53 
3. 60 
22 
11 
16 
4 
E. 
August 
80 
88 
29 
71 
20 
10. 38 
5.53 
19 
14 
12 
5 
E. 
September 
80.6 
91 
n 
71 
30 
13. 66 
3. 76 
20 
13 
11 
6 
SE. 
October 
79. 5 
90 
10 
68 
1 
10. 21 
2. 73 
20 
6 
12 
13 
SE. 
November 
78.8 
88 
29 
70 
19 
11.81 
2. 57 
16 
ii 
14 
5 
SE. 
December 
76 
88 
2 
65 
26 
2. 10 
.49 
10 
18 
11 
2 
NE. 
January 
75.6 
86 
i 
68 
15 
3.93 
.75 
21 
13 
18 
0 
E. 
February 
75. 7 
86 
25 
66 
19 
2. 13 
.70 
13 
12 
15 
1 
E. 
March 
75.8 
89 
31 
67 
7 
1.57 
.71 
11 
14 
16 
1 
E. 
April 
77.5 
93 
25 
68 
8 
5.92? 
1.91 
19 
11 
17 
2 
E. 
