90 



ON THE TOPOCIKAPHY AND CIEOLOGY 



CHAPTER V. < 



I N T R U S I V E R 0 C K S . • 



The Cretaceous rocks which form the basis of the Ishiud are elevated into a series 

 of folds and undulations by an immense mass of granitoid rocks, which apj)ear as a 

 " massive eruption occupying the heart of the range and forming a belt, sometimes 

 entire, but more usually consisting of two, three or more parallel lines. Although 

 the Pico del Yac[ui is entirely composed of this rock it is the exception rather than 

 the rule to find it making up the higher points. It seems to have forced up the meta- 

 morphosed slates in great masses, and shows itself at their bases or along their lianks, 

 exposed as often by denudation as by actual outflow. Its general direction is much 

 more to the northwest, as a whole, than the axis of elevation* of the chain; so that 

 although its western half coincides with the ti'end of the mountains its eastern end 

 bends far to the southward. This eastern end also covers a greater width than any 

 other part, extending continuously or nearly so from the head of the Jaina to far 

 down on the ]Srizao. In the mountains northwest of the Peak of the Yaqui the 

 eruption has taken place along three parallel lines, throwing up the slates into the 

 high summit-ridge to the south, and making two marked sjaiclinal axes, on the edge 

 of one of which is the tall Pico del Gallo, The most northern of these exposures is 

 quite near the northern base of the range, and it is more than probable that this is 

 due to denudation, since the slates, although uptilted at high angles, are not raised 

 to an altitude at all comparable with those further south. 



On some parts of the margin of the eruption the overlying rocks have suffered 

 much fracturing, and the subjacent molten matter has been injected into the fissures, 

 sometimes to a distance of several miles from the nearest surface exposure of the 

 parent mass. This is notably the case on the Jaina and Nizao Rivers. On the former 

 stream, at and even below the mouth of Madrigal Creek, the jaspery slates are seamed 

 with dykes of [ill sizes li'om a mei'e thread up to many feet across ; while the nearest 

 exposure of the mass is near Catare, eight miles fiu'ther up the river. So on the 

 ]S[izao, dykes occur almost as far west as Maniel, while the western margin of the 

 main eruption is between the !Nizao River and its tributary, the Majoma, which enters 

 it from the east. 



A remarkal)le feature of the intrusive rocks of Santo Domingo is that, although 

 all are of Tertiary age or at earliest, some of them may date back into the latest 



* Kicbtbofuu, NiiUual System of Volcanic Rocks, p. 'J. 



