Vol. 62.] CATARACTS OF THE RIVER MADEIRA, ETC. 113 



The original rock at Theotonio appears to have been a glass} 

 aegirine-augite-olivine-basalt containing minute steam- 

 cavities. These last furnish evidence that, at the time of the con- 

 solidation of the basalt, the syenite into which it was intruded was 

 at no great distance below the surface, although there is every reason 

 to believe that originally it solidified at a considerable depth. A 

 considerable amount of erosion must have occurred, and a long 

 period of time must have elapsed in the interval. 



Another dyke (M 10) has a minutely- granular appearance in 

 hand-specimens. Under the microscope, it proves to consist of a 

 holo«rystalline mixture of felspar and hornblende, the amount of 

 the latter being about a fifth of that of the former. The pre- 

 dominant felspar appears to be microperthite, though microcline, 

 microcline-perthite, and albite-oligoclase are also present. Most 

 of the felspars vary between *1 and '2 millimetre in diameter, but 

 occasional crystals reach or exceed a millimetre. Quartz is only 

 represented by a few small sporadic grains. . 



There appear to be two generations of hornblende : the older has 

 a green and yellow pleochroism, is more or less equally developed 

 in all directions, and sometimes measures a millimetre across. The 

 angle between the direction of extinction and the vertical axis 

 reaches a maximum of about 18°. 



The hornblende that crystallized out later shows pale and dark 

 shades of green. It occurs in long needles or thin plates ; these 

 are sometimes independent, and exhibit sections which occasionally 

 reach a millimetre in length, and sometimes are in crystalline 

 continuity with the older hornblendes, although extinguishing at a 

 lower angle, which does not appear to exceed 6°. In some cases, 

 a number of these later hornblendes show parallelism with one 

 another — being all connected together, so as to form a branching 

 comb-like structure, of which one of the older crystals may 

 constitute a part ; the connection may not, however, be visible in 

 the slide. 



There are a few flakes of dark mica, and numerous small prisms 

 of apatite. Sphene and magnetite are not uncommon. 



The rock may be described as a fine-grained syenite rich 

 in soda. 



Except in the immediate neighbourhood of the river, the rock 

 of the ridge that gives rise to the cataract is altered for a depth of 

 several feet to an argilla ceous m aterial , probably laterite. A similar 

 but less extensive change has taken place at some of the other 

 cataracts. 



Beyond the cataract of Theotonio we passed that of Macacos, but 

 did not land, and before long reached the last cataract of the 

 Madeira, that of Aroya or Aroeira. It was at one time known as 

 Sao Juan, but is usually referred to at the present day as the 

 cataract of S an to Antonio, from the settlement of that name in 

 the immediate neighbourhood. 



Q. J. G. S. rTo. 245. i 



