PETROGRAPHICAL NOTES. 11 



sides these there are other dioritic and granitic rocks, the 

 exact relation of which to the others coukl not be clearly 

 established. In the types described above, hornblende is 

 either absent, or else only appears as indefinite remains of 

 magmatic resorption. 



3. Specimens taken near the Pacific, on the south side of 

 Pnnta Banda, opposite the proposed hotel, show a grayish- 

 brown groundmass, with a few small feldspar and horn- 

 blende prisms. Under the microscope the feldspars are 

 seen to be plagioclastic, presumably oligoclase; the horn- 

 blende crystals are decomposed to chloritic aggregates. 

 The groundmass is a microcrystalline feldspar-quartz aggre- 

 gate, which has a microfluidal structure, indicated by alter- 

 nating lighter and darker bands, and caused by the une- 

 qual distribution of opacite; the grain of the groundmass 

 also varies between certain limits in the difierent bands. 



4. The most common type appears to be that exhibited 

 by the specimens from the nortli side of the Punta Banda 

 Range, south of the estuary and El Ma^ieadero. It is a 

 greenish rock with a crystalline groundmass, in wdiicli are 

 imbedded large white tabnlar feldspar crystals and small 

 prisms of hornblende. Under the microscope the constit- 

 uents of the first generation are, 1. Idiomorphic plagioclase 

 of tabular habit. Isolated fragments give, wdth Boricky's 

 test, abundant Na — less so Ca reaction, and certainly are 

 oligoclase. 2. Smaller quartz grains, partly idiomorphic, 

 partly rounded, filled with fluid inclusions with moving 

 bubbles; also, gas inclusions. 3. Hornblende prisms de- 

 composed to chlorite and bastite. 4. Magnetite. 5. Zircon. 



The holocrystalline groundmass consists of relatively 

 large grains of quartz and unstriated feldspar connected by 

 an intimate granophyric structure. 



According to Rosenbusch the rock would be designated as 

 a quartz-liornhleiuk granophyrite. 



