BERKEY, GEOLOGICAL REC0NN0I8SANCE OF PORTO RICO 67 



geology. In addition to tlie regular pliotograplis, a number of photomi- 

 crographs have been made from the thin sections of typical rocks. 



Some of this material will finally serve as a foundation for the geo- 

 h)gical section (if what it is hoped may Ix'conic a Natural History Museum 

 of Porto Rico. 



Illustrations 



cross-sections 



The note ])ooks of tlie party and field maps carry a record of field de- 

 terminations and detail of structural relations and comments greatly in 

 excess of what can be published in sudi a report. They arc the ])roperty 

 of the organization iind -.wv of partifular service as guides in plajming 

 furtlier woi'k and in giving each lU'W investigator his bearings, together 

 w ifli sonic suggestions .-iliont tlic cliaiiictci' of his own district or the dis- 

 tribution of (lata bearing upon his special iuNcstigatiou. The note books 

 contain observations along some of the principal roads in sufficient detail 

 to serve as a foundation for complete generalized geologic cross-sections 

 of the island on two esi)ecially important lines. 



Cross-sections, therefore. hav(> been drawn to illustrate the kind of sur- 

 face relief, the grade of tlu' rond. the kinds id' rocks or rock formations 

 and the geologic structurnl rtdations. and arc I'l'produced to accompany 

 this report. An inunense amoniil (d' detail is necessarily omitted or com- 

 hincd into giMicralizations in oi'der to bring the sections within the scope 

 of a publication of this kind. It is judged that some of these details will 

 i)e suitable illustrati\(' matter foi' future i'e]>orts hased on studies of spe- 

 cial districts. One of the sections is hased on data gathered along the 

 roail from Ponce to Arcciho. The line is di'awn from Arecibo to Ponce 

 diiV(;t and the data are [)rojecti'd to this line. This method tends to 

 obliterate the windings of the I'oad and secure practically normal propor- 

 tions and i(dati\(' positions for the associated formatioiuil units. The 

 other section line is drawn directly from San Juan Point to Santa Isabel. 

 By projecting to this line all the data gatliered on the Bayamon-Comerio- 

 Barranquitas-Coamo roa<l a great deal more elimination of road curves is 

 accomplished than in the other section, and it makes the grades of the 

 road look somewhat abnormal by reason of this shortening of road dis- 

 tance of certain large curves, but on the whole the relations are shown 

 without special difficulty except that attendant uiion the need of general- 

 izing the minor structural detail. 



