36 



University of California. 



[Vol i. 



from which it might have been derived by process of alteration 

 seems to favor the latter possibility. In this case, however, we 

 should expect that the water yielded in the closed tube would be 

 derived from the limonitic pigment, which is not a part of the sili- 

 cate molecule. To test this the following experiment was made: 

 A small quantity of the mineral, in minute fragments, was di- 

 gested in hydrochloric acid till the brown color was discharged, 

 and the iron, so far as could be judged, was leached out. Some of 

 the fragments were then tested optically, and were found to give 

 the same reactions as before treatment. The fragments were then 

 dried, powdered, dried again for several hours at ioo° C, and then 

 tested for water in closed tube, with an affirmative result. The 

 experiment shows that, although a portion of water is doubtless 

 present in the form of limonite, there is also a portion which re- 

 mains after the latter is removed, and is part of the silicate molecule. 



The Sunium Point Eruptive Area. — A glance at the accom- 

 panying geological map will show that the carmeloi'te in the vicinity 

 of Carmelo Bay occurs in six separate patches. The map being a 

 sufficient account of the distribution of these patches and of their 

 extent, we will proceed, without dwelling on these matters, to de- 

 scribe the rock in detail, taking up the different areas or patches 

 in succession. The most interesting of these areas, if not the largest, 

 is that which occupies Sunium Point and the shore to the north- 

 east and east of it, to the extent mapped. The extremity of Sunium 

 Point is composed of a dense, compact rock, having a bluish gray 

 aphanitic base, throughout which are scattered numerous small 

 phenocrysts of feldspar and iddingsite, the latter of a deeper red 

 color than it ordinarily presents, and showing the usual lustrous 

 cleavages. The size of these phenocrysts of iddingsite rarely ex- 

 ceeds a millimeter in length, and they commonly range from about 

 .25 to .5 mm. The phenocrysts of plagioclase may be as much as 

 2 mm. in length. Under the microscope the ground-mass of this 

 rock appears to be a fine feltwork of lath-shaped plagioclase, with 

 numerous minute granules of magnetite, and of a rather brightly 

 polarizing mineral (pyroxene). In addition to these there are also 

 numerous rod-like microlites, which with low powers appear to be 

 opaque, but which under higher powers are in some cases feebly 



