124 JOURNAL OF THE [April, 



sirable mode of treatment, and he (Mr. Cox) felt sure that this 

 Society would be glad to receive the new set of photographs 

 which Dr. Van Heurck said he had sent, in illustration of his 

 new and improved method. 



Mr. P. H. Dudley being unavoidably absent, a communication 

 from him, explaining his exhibits as announced in the pro- 

 gramme, was read from the chair, as follows : 



ROCK-SECTIONS, FROM THE ISTHMUS OF PANAMA. 



"The section of rock (Wacke) from the Bujio quarry of the 

 Panama railroad, is of considerable interest, from the fact of its 

 extensive use for heavy masonry by the Railroad Company on 

 the Isthmus. 



"The rock is friable and porous, as a glance at the slide in- 

 dicates. The fragments composing it are only feebly united, 

 while some portions have undergone decomposition. Most of 

 the Olivine has disappeared. Pyroxene crystals are present, 

 sufficient in number to deepen the color of the rock, and to in- 

 crease its specific gravity, the latter ranging from 2.3 to 2.5. 



" In this climate exposed blocks of such rock would not 

 withstand one winter's freezing and thawing, without extensive, 

 if not complete disintegration. In a tropical climate it stands 

 much better, and of necessity the Panama Railroad Company 

 used it for all of their bridge abutments and other masonry, in 

 which it has rendered good service, when laid in cement and 

 kept well pointed. When used for road metal or ballast, for the 

 railroad, it is rapidly converted into mud in the 'wet season.' 



" Sections of Pebbles. — The pebbles found in the Chagres 

 River, at Gorgona, are from both igneous and sedimentary 

 rocks, the former being the most abundant. The sedimentary 

 pebbles were all argillaceous, the corners being well rounded. 



"The pebbles from the igneous rocks were not, as a rule, as 

 well rounded, many of them being quite angular. 



" In slide No. 5 of the programme are a number of sections 

 of crystals (Orthoclase), each seeming, at first sight, to form a 

 nucleus for the surrounding material. The want of sharpness 

 of many of the corners of the crystals, and the fact of several 

 being broken, favor the hypothesis, that they were undergoing 

 a process of dissolution, which was subsequently checked. 



