BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 67 



geology. In addition to the regular photographs, 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- 

 logical section of what it is hoped may l)ecome a Natural History Museum 

 of Porto Eico. 



Illustrations 



cross-sections 



The note books of the 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 such a rej^ort. They are the property 

 of tlie organization and are of particular service as guides in planning 

 further work and in giving eac]i new investigator his bearings, together 

 with scmie suggestions about the cliaractcr of liis own flistrict or the dis- 

 tribution of data bearing upon his special investigation. 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 especially important lines. 



Cross-sections, therefore, have been drawn to illustrate the kind of sur- 

 face relief, the grade of the road, the kinds of rocks or rock formations 

 and the geologic structural rehitions, and are re])r()du(*ed to accompany 

 this rejiort. A.n immense amount of detail is necessarily omitted or com- 

 i)ined into generalizations in order to bring the sections within the scope 

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

 be suitable illustrative matter for future reports based on studies of spe- 

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

 road from Ponce to Arecibo. The liiie is drawn from Arecibo to Ponce 

 direct and the data are projected to this line. This method tends to 

 obliterate the windings of the road and secure practically normal propor- 

 tions and relative positions for the as.S()ciated formational units. The 

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

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

 Barranquitas-Coamo road 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 upon the need of general- 

 izing the minor structural detail. 



