316 C. W. HAYES — GEOLOGY OF NICARAGUA CANAL ROUTE 



surface lava flows, and intrusive masses of rather coarsely holocrystalline 

 rock which did not reach the surface hefore cooling. 



Massive igneous rocks. — The principal varieties of igneous rocks which 

 are found between the lake and the Caribbean are augite nndesiie, olivine 

 basalt, hypersthene basalt, and dacite. Of these four varieties the first 

 three are very similar in appearance, and can not ordinarily be distin- 

 guished with certainty in the field. They are dark bluish gray to 

 black in color, generally fine grained, but often containing certain min- 

 erals, as olivine and feldspar, which can be readily distinguished with 

 the unaided eye. They are generally compact and heavy, though a 

 well marked vesicular structure characterizes some portions of the 

 basalt. The red clay which is the final product of their decay contains 

 numerous residual boulders of the fresh rock covered with a thin 

 ochreous crust. 



The dacite is light gray in color, and is made up of abundant quartz 

 and feldspar crystals embedded in a fine grained or glassy, gra}'' ground- 

 mass. It is lighter than the trap rocks, and is considerably softer even 

 when entirely unweathered. The dacite contains numerous fragments 

 of darker basic rocks. It doubtless reached its present position as a 

 lava flow, and these inclusions are fragments of the underlying rock 

 which were picked up and incorporated in the molten mass during its 

 passage through the lower formations to the surface. Many of them are 

 a soft greenish rock exactly like the tuff" on which the dacite rests. The 

 presence of these included fragments of a different rock is doubtless the 

 reason the dacite was called conglomerate in the Canal company's east- 

 ern divide sections. Of the fragraental igneous rocks two classes may 

 be made, according as their igneous or sedimentary characteristics are 

 the more prominent. In the first class are included the tuffs which 

 form the western portion of the eastern divide, passing under the dacite 

 at an angle of about 5 degrees. This tuff" is related to a basic lava, either 

 andesite or basalt. It has a dark greenish color and very fine grain. It 

 is soft and talcose, and on exposure to the air the cores generally crumble 

 into small fragments. While this tuff" owes its kaolin-like character to 

 the decomposition of a basic glass, it was probably never a hard rock. 

 The pressure to which it has been subjected since its deposition has ap- 

 parently not been sufficient to produce complete induration. 



The extent to which these rocks have weathered has been alread}'' 

 pointed out, but may be referred to again in explanation of the difficulty 

 which has been experienced in determining the relations of the various 

 members'of the volcanic formations. All weather to a red clay, and ex- 

 posures which afford any indication of the original character of the under- 



