METAMORPHIC AND IGNEOUS ROCKS. 

 Specimens of granular intrusive rocks — Continued. 



85 



DIKE ROCKS. 



Many of the specimens are either described as occurring in dikes or, as 

 their texture indicates, are obviously dike rocks. Some others are only 

 tentatively classified as dike rocks in the absence of adequate information 

 regarding their mode of occurrence. 



Silicic dike rocks. — The most common sort of silicic dike rock in the col- 

 lection is a whitish granite porphyry containing phenocrysts of quartz and 

 albite and sparse vestiges of some altered ferromagnesian mineral in a 

 groundmass of albite, quartz, and orthoclase. Specimens of such rock 

 were collected in the Provinces of Santiago and Pacificador. An aplitic 

 rock (D. C. 27a) from the Province of Santiago consists essentially of feld- 

 spar, abundant quartz, and a moderate amount of hornblende. The feld- 

 spar is albite or sodic anorthoclase. Both this aplite and the granite 

 porphyry may be genetically related to the quartz diorites. 



Intermediate dike rocks. — A dark-green sheared and highly altered diorite 

 porphyry (D. C. 23b) was found in the Province of Santiago, and a very 

 fresh gray diorite porphyry (D. C. 83) from the Province of Azua contains 

 phenocrysts of labradorite, augite, hornblende, and biotite. 



Mafic dike rocks. — The collection contains several specimens of diabase, 

 which may be dike rocks, though some of them may be lavas. They are 

 from the provinces of Azua and Santiago. Two specimens (R. 7 and D. C. 

 56a) have the texture of diabase but contain hornblende in place of augite ; 

 in D. C. 56a the feldspar is near albite and probably secondary. 



Some other dark greenish dike rocks containing augite but not diabasic 

 in texture are listed as lamprophyres. They were found in the provinces 

 of Samana, Monte Cristi, and Santo Domingo. They are considerably 

 altered and in at least one (D. C. 68b) the feldspar is albite. 



