98 DR. J. W. EVANS ON THE ROCKS OE THE [Feb. I906, 



river being far too sluggish to carry stones of such size. They 

 must be derived from strata flanking the crystalline rocks on 

 the south-west, possibly from the reef of thirteen rocks above 

 Esperanza. 



The occurrence of marine beds in this region of alluvium and 

 crystalline rocks is of great interest. I may mention that I found 

 pebbles of silicified oolite in the Upper Paraguay, at Santa Cruz, 

 Barra dos Bugres. 1 



(3) The Cataracts of the Mamore. 



Leaving the Esperanza Cataract behind me, I descended the river 

 to Villa Bella, which is situated on the tongue of land between the 

 Beni and the Mamore. Thence I made a short journey up the latter 

 river, in a canoe, to the rapids of L a g e s, where broad rock-pavements, 

 which give their name to the cataract, are exposed when the 

 river is low. 



Megascopically this rock (M 2 & 3) appears to consist of quartz 

 and felspar speckled with flakes of black mica. The size of the 

 constituents varies from place to place, but the grain is always 

 rather fine. There is distinct banding, which has more the 

 appearance of flow-structure than of the foliation of gneiss. The 

 rock splits somewhat easily, but in a direction which is parallel 

 rather with the surface than with the planes of banding. Its specific 

 gravity is 2*66. 



A microscope-section of the coarser (M 2) material (PI. Y, figs. 1 

 & 2) shows abundance of felspar, including microcline — sometimes 

 with microperthitic inclusions, and plagioclase with lamellar twinning. 

 The latter appears to be intermediate between albite and oligoclase ; 

 it shows in some cases inclusions the boundaries of which are parallel 

 with those of the host; they have lower refraction and birefringence, 

 and are probably orthoclase. Quartz occurs, both independently and 

 as inclusions in the felspar ; as in the rock of the Esperanza Cataract, 

 it appears to have crystallized out before the latter mineral. The 

 sections often show crystallized outlines, but are sometimes only 

 rounded blebs. Similar rounded inclusions of felspar, especially 

 microcline, also occur. In some cases, the felspar appears to have 

 crystallized in the cavities of the quartz. Micrographic intergrowth 

 is fairly common. There is much brown and green mica representing 

 biotite in different stages of alteration into chlorite. Epidote, 

 apatite, and magnetite are also present. 



The finer-grained specimens (M 3) are similar, but show crystals 

 of yellowish-green hornblende, with an extinction-angle (between the 

 direction of slower vibration and the cleavage) which reaches a 

 maximum of 29°. The colours of different directions of vibration 

 vary between yellowish-green and dark green. The largest quartz- 

 and felspar-crystals measure about a millimetre in diameter, but 

 numerous small quartz- and felspar (microcline)-grains occur, both 



1 < The Geology of Matto Grosso ' Quart. Journ. Geol. Soc. vol. 1 (1894) p. 91. 



