THE ROCKS OF THE WASHOE DISTRICT. 41 



about an incli from the edge the intrusive ixjrplijritie rock has an exces- 

 sively fine grain and close texture. In consequence of this physical char- 

 acter it has resisted decomposition, and close to the contact is very fresh. 

 Here it contains fine brown hornblende, Ijut at a distance of half an inch 

 from the contact, the texture, as seen under the microscope, becomes coarser 

 and more open, and green fibrous hornblende makes its appearance. Cer- 

 tain hornblende indi\ iduals are luown towards the center, but <>reen and 

 fibrous near the edges and along cracks, and the dividing line is such as to 

 leave no doubt that in this case the green fibrous modification is to be 

 regarded as an alteration-product of the brown dense variety. This occur- 

 rence strongly confirms the indications of such a transfoi-mation mentioned 

 in describing the granular diorites ' 



Structure of porphyritic diorites. — No spccial tendeucy to parting in any direction 

 is perceptible in the porphyritic diorites, but they vary in coarseness of 

 grain and general aj)pearance nuich more than the granitoid diorites. To 

 the south of Bullion ravine there are small localities where the rock is dis- 

 tinctly brecciated; and in the ravine west of the Impoial there is a small 

 occurrence of excessively fine-grained diorite, with a closely laminated 

 structure, not unlike a calcareous slate. Similar spots are found in the 

 diabase and the andesites, but in no case was any explanation apparent from 

 the character of the surrounding masses. Some such appearance might 

 ensue in a pasty mass if its composition were locally altered and its fusi))ility 

 increased, say bj' the presence of a fragment of calcareous rock. There is 

 no relation between the direction of the lamellfe of these spots and the 

 general fissure system. Where decomposition has proceeded far enough 

 for a diffusion of chlorite througli the rock to take place, the granular texture 

 of the groundmass is much obscured, and the similarity between it and 

 certain partially decomposed andesites is great and misleading. A laro-e 

 portion of what was supposed to be pi-op)lite in th.e WAsnoic Disteict, is 

 porphyritic diorite in this stage of decomposition. Disintegration sometimes 

 accompanies decomposition of the rock, but an astonishing coherence is often 

 maintained when scarcely a particle of unaltered mineral is left. 



Diagnostic points. — It Is ofteu vcrj dif!icult to distinguish between partially 



' Confer Kosenbusch, Pliysiog. der Mill. u. Gcst., Vol. II., p. :i33. 



