100 QuiCKsiLVKi; i)i;p()siTs op tiik rAciFu; blopk. 



find characteristic distinction between pseudodioritu ami pseiidodiabase, as 

 seen nnder tlie microscope, excepting the character of the bisihcate, and 

 it is only when the bisihcate is unusually aluindant that tlio two rocks can 

 bo told apnrt without the microscope. 



An interesting pseudodiorite is No. 17'J, Ivnoxville. Tiiis is a fine- 

 grained, crystalline, dark-green rock, in which amphibole is visible macro - 

 scopicallv. Under the microscope it is seen that the rock is a porphyry, 

 containing large grains of hornblende in a fine-grained, colorless ground- 

 mass. When well exposed this groundmass shows a faint net-work of green- 

 ish lines, which, judging from the form of the net, represents the outlines 

 of the original clastic structure. Ci'acks also intersect the groundmass, 

 and these often radiate from the porphyritic hornblendes in a very peculiar 

 manner, as if the hornblendes had expanded forcibly while forming. T!ie 

 groundmass is very fine-grained, the individuals ranging from 0.01™™ down- 

 wards. 



Polysynthetic microlites were not detected, bnt the niatei'ial precisely 

 resembles that in another rock from the same district (No. 105, Knoxville), 

 which was isolated and found to have a specific gravity of 2.64, to contain 

 59.14 per cent, of silica, and to correspond qualitatively to andesine i:i 

 composition. 



The hoi-nblende is of a brownish-green color and forms grains read i- 

 ing half a millimeter in length. In the hornblendes are small shreds and 

 patches of glaucophane. In other pseudodiorites (for example, No. 183, 

 Knoxville), there is more glaucophane and single crystals of amphibole 

 niav be seen, blue at one end, green at the other, and of intermediate tints 

 in the middle. The glaucophanic pseudodiorites form a link between the 

 granular, crystalline rocks and the glaucophane schists. This slide shows 

 ilmenite and leucoxene, but no zoisite was detected. Tliere runs through 

 the slide a vein which is filled with chlorite and a colorless mineral of un- 

 certain character. 



The hornblende in the pseudodiorites is sometimes so abundant as to 

 form much the greater part of the rock, wdiich may then be considered as 

 an amphibolite One of the best examples of this kind is No. 56, Knox- 

 ville. It is composed almost exclusively of long, slender, greenish crystals 



