﻿248 SMITH. 



the feldspar. Clearly it is a decomposition product of the feldspar. 

 Chlorite is also in evidence in irregular patches between the larger 

 minerals. It is almost impossible to specify the particular composition 

 of the plagioclases, as no good prism faces nor sections cut normal to 

 the albite twins were found. However, one zonally built plagioclase was 

 investigated with the result that the center was found to have about the 

 composition of oligoclase, with albite forming the outer shell. The 

 only name I feel warranted in assigning to this rock is that of feldspar 

 porphyry. The rock is far from being fresh. Plate IV, fig. 2, is a 

 photomicrograph, with ordinary light, of this rock. 



BEXGUET NO. 103. BASALTIC TUFF. 



Macroscopic. — This rock is apparently made up of the same materials 

 as Xo. 101. although it is in a much finer state. The groundmass is 

 possibly somewhat more prominent and hence the rock is more nearly 

 of its color, which is a chocolate-red. The largest fragments in this 

 specimen do not exceed 3 millimeters in length. These is no efferves- 

 cence with acids. 



Microscopic. — "\Yhen we study the slides of this rock the supposition 

 that this is simply a finer state of Xo. 101 seems to be borne out, for 

 few rock aggregates are seen as in Xo. 101, merely a loose textured 

 aggregate of minerals, quite fragmental in appearance is visible. The 

 grains are all more or less rounded, although not all to the same degree 

 as in sandstone or quartzite. 



The most abundant mineral of the rock is pyroxene, greenish-yellow 

 in some sections, almost colorless in others, yet it is not pleochroic. It 

 occurs as mere fragments, irregular and rounded, usually showing pris- 

 matic cleavage. Olivine also in rounded grains, colorless and possessing 

 irregular cleavage cracks, can be seen in several places in the slide. 



Magnetite is probably the most abundant mineral after pyroxene. It 

 is sometimes seen inclosed in the pyroxenes. Feldspar is reduced to a 

 minimum in these slides, usually occurring as very small grains and in 

 one case only showing any distinct twin striations. ■ In some of the 

 slides fragments of actinolite were seen. These were generally pleochroic. 

 The presence of both actinolite and augite is assured, as one portion of 

 a basal section was found. These minerals are frequently seen to be 

 surrounded by opaque masses of hematite and limonite. 



Remarks. — The rock is clearly pyroclastie and owing to the presence of olivine 

 would more properly be called a basaltic tuff rather than an andesitic tuff, as in 

 the case of Xo. 101. 



The large amount of quartz would presuppose an acid magma, far more acid 

 than any which has ever been encountered in igneous rocks of this region. If 

 this be considered a graywacke the quartz could naturally be segregated on the 

 beach so as to give its present richness in silica. The slides show considerable 



