DETAILED DESCRIPTlOISr OF SLIDES. 97 



fluid inclusions, many of them of unusually large size. Some are dihexahe- 

 dral in shape ; the bubbles of the smaller ones are active, and some contain 

 excellent salt cubes. The proportion of salt to water seems to be very high; 

 for on heating the slide to about 70° C, the only effect produced was to 

 round the edges of the cubes. The bubbles did not grow perceptibly 

 smaller at this temperature. 



The slide is further remarkable for containing what appears to be 

 tourmaline. One small patch dichroizes between black and clear brown. 

 The mineral exhibits scarcely any structure, but there are traces of what 

 appear to be cleavage cracks parallel to the direction of extinction. No 

 distinct interference figure could be obtained. The lack of structure and 

 the absolute extinction of the ordinary ray seem to separate this substance 

 from hornblende ; to mica it bears no resemblance. 



PORPHYRITIC DIOKITE. 

 Slide 421. Center of Cedar Hill Ridge. 



Fresh porphyry. — The mass of porphyritc forming Cedar Hill is ver}'- uneven 

 in composition, and, for the most part, greatly decomposed. Near the high- 

 est portion, however, is a small quantity of a comparatively fine-grained 

 variet}', which, from one of the accidents so common in regions of decom- 

 position, has escaped nearly unaltered. Macroscopically it is a dark, leaden- 

 gray rock, rather fine in texture, and exhibiting porphyritical crystals of 

 feldspar and hornblende. Under the microscope it is seen that these min- 

 erals are separated out in a groundmass of tolerably fresh feldspar micro- 

 lites, and magnetite, to which the dark color of the rock is due. Numerous 

 colorless apatites form the only other prominent mineral ingredient. The 

 hornblendes are almost wholly undecomposed. They are of a slightly 

 greenish-brown color and fairly well-cry.stallized. Most of this mineral 

 occurs in crystals of large size, but there are a few minute crystals and 

 crystalline fragments interspersed through the groundmass. The horn- 

 blende is dense, though in many cases the cleavages are well developed, 

 and one crystal even contains fluid inclusions (1 0-24 J). There is no tend- 



