PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 397 



Apotrachyte. Basalt.] 



No. 521. APOTRACHYTE. (Breccia.) 



"Is an irregular thrust-up rook, appearing in the beach near the west end of No. 520. Its bedding is 

 distorted. It almost appears conglomeritic." 

 Kef. Annual Report, x, pages 38, 114. 



Mey. The hand sample shows a breccia composed of rock which is somewhat 

 similar to No. 520, but is more decayed and darker colored. Occasionally small spots, 

 apparently amygdules, can be seen in the rock. It is seamed through and through 

 by fine pinkish and gray veinlets, and the cementing material of the whole rock is a 

 white, very finely crystallized silica, with some calcite. 



Mic. The section was evidently made from one of the larger fragments of the 

 hand specimen. It shows small porphyritic feldspars, considerably altered and clouded 

 and frequently holding chlorite in irregular areas in a groundmass composed of small 

 poikilitic areas of quartz holding minute, cloudy, feldspathic grains, and in all 

 respects resembling the groundmass of many of the devitrified rocks already described. 

 The species of the porphyritic feldspars was not determined. Pyrite and calcite also 

 occur. The rock can be referred to an apotrachyte. 



One section. 

 Age. Cabotian. 



Remark. From the field description it seems possible, though not certain, that 

 we have here a deposit of tuffaceous material. The hand specimen, however, would 

 not warrant us in certainly referring this rock to a volcanic f ragmental. u. s. G. 



No. 522. BASALT. ( AmygdaMdal. ) 



Near the top of the bluff in the bay first north of Splitrock point. 

 Ref. Annual Report, is, page 30; Annual Report, x, page 40. 



Mey. A very fine-grained, dark brown rock, with amygdules of laumontite and 

 calcite. The rock appears considerably decayed and contains many indistinct small 

 areas of chlorite. 



Mic. The section is made up of minute lath-shaped feldspars in a mass of 

 alteration products, hematite, miiyitetite and chlorite. The feldspars are somewhat 

 altered. A considerable number show albite twinning and equal extinction angles 

 on either side of the composition face running up as high as 23, indicating lahra- 

 dorite. What the original nature of the rock, aside from the feldspars, was, is 

 uncertain, but it seems most probable that it was largely glassy. 



Two sections. 



Age. Cabotian. u. s. G. 



No. 523. BASALT (?) 



From the base of the bluff from which No. 522 was taken. . 



Ref. Annual Report, ix, page 30; Annual Report, x, page 40. 



