DETAILED DESCRIPTION OP SLIDES. 101 



expense of the chlorite. This is well illustrated in Figs. 6 and 7, Plate IT., 

 especially in Fig. 7, where the growth of the epidote into the chlorite is 

 accurately and clearly shown. It is very noticeable that, as has already 

 been mentioned, the chlorite at the edges of the hornblende sections fre- 

 quently remains undecomposed longer than the interior mass. The behav- 

 ior of this peculiarly arranged chlorite seems to indicate a greater density, 

 and consequentl}'' a greater resistance to decomposition, than is possessed by 

 that with spherolitic structure. In a majority of cases the decomposition 

 of chlorite into epidote begins toward the center of the section, but there 

 are many exceptions. It is probable that the veins and patches of epidote 

 not connected with the hornblende sections have also been formed from 

 chlorite, for the latter appears to be the more soluble mineral. There is 

 evidence too, from other slides of the same rock, that, as decomposition 

 proceeds, the chlorite is replaced to an increasing extent by epidote, etc. 

 The chlorite in this rock also decomposes into quartz, calcite, and limonite. 

 Whether epidote, too, undergoes the same decomposition is uncertain. 

 Forming, as it does, masses of irregular granules and imperfect prisms, it 

 would be difficult to show that it had been encroached upon in any given 

 case by quai'tz and calcite, and had not formed simultaneously with 

 them. 



There is no augite in this rock, but a little (5-23) mica, which, like 

 the hornblende, has been converted into chlorite and epidote. The feld- 

 spars still show twin striations, but are considerably decomposed, and under 

 high powers the mass is seen to be porous or even spongy. Particles of 

 chlorite, epidote, quartz, and calcite are disseminated through the feldspai's. 

 In some of the freshest portions fluid inclusions may be detected. The apa^ 

 tites are all colorless, and sharply crystallized. Fig-. 18, Plate III., shows a 

 curious case, in which an intrusive bay of groundmass has reduced an apa- 

 tite section to the form of a horseshoe. There is a considerable amount of 

 pyrite in this rock (which occurs near ore), but only a trifling amount of 

 magnetite. The groundmass shows gray, semi-opaque markings, not dis- 

 similar to stippling. This appearance is caused in part by particles of cal- 

 cite, etc., but close examination shows that it is largely due to the spongy 

 structure mentioned above. 



