DETAILED DESClilPTION OF SLIDES. 125 



substitution of calcite for chlorite begins, the conversion of hornblende 

 into chlorite seems to cease, and this slide shows many bright, fresh frag- 

 ments of hornblende embedded in calcite. There is no indication that the 

 hornblende tends to pass directly into calcite. Were such a process going 

 on, we should find denticles of calcite penetrating the hornblende. Neither 

 do I see any reason to suppose that the epidote in this slide passes into 

 calcite; it appears rather to give place to clouds of dark-colored opaque 

 matter, which may be oxides or earthy silicates. In this and the other slides 

 of andesite which contain hornblende free of black borders, I see no indica- 

 tion that magnetite, or anything resembling magnetite, results from the 

 decomposition of hornblende. In the black-bordered hornblendes I have 

 often suspected such a change, but I see no way of proving that the particles 

 in question may not have formed a part of the original border. A horn- 

 blende in process of decomposition is shown in Fig. 2, Plate II., from this 

 slide. A portion of the augites are also partially converted into chlorite, and 

 in the pseudomorphs epidote is certainly developed parasitically. The large 

 feldspars are tiiclinic, and give angles of extinction answering to labradorite. 

 In one of them a bubble-bearing and only partially devitrified glass inclusion 

 was observed. The raicrolitic groundmass contains some magnetite, pyrite, 

 and ordinary apatite 



Slide 116. Crown Point Eavine. 



propyiitic variety. — This is a vcrv bhick, fine-grained rock, which, however, 

 proves, under the microscope, to derive its color from an iinusual amount 

 of magnetite in the groundmass. The hornblendes are altered to chlorite 

 and epidote, and only a few sections have retained characteristic outlines. 

 It is evident that the chlorite preceded the epidote, and in some cases the 

 encroachment of the latter can be very well observed. A portion of the 

 chlorite has been replaced by quartz and calcite. 



As I shall have occasion to refer to slides of the Fortieth Parallel Sur- 

 vey collection from Crown Point Ravine, and from the South Twin Peak, 

 it would be an unnecessary repetition to say more of my own sections from 

 these localities than that there is no notable difference between them and 

 those described by Professor Zirkel. 



